Proceedings of the American Society for Composites — Thirty-fifth Technical Conference
Fabrication and Characterization of Sandwich Composites Containing Micro- and Nano-Channel Architectures
Lightweight composite materials are desirable in the transportation and construction industries, specifically those which may utilize both low thermal conductivities and strong mechanical properties for application as structural, thermally insulating composites. A manufacturing technique has been developed which permits the introduction of micro- and nano-scale hollow channels into a polymer matrix, with demonstrated control over both the channel volume fraction and channel orientation. Systematic variation of the volume fraction and/or orientation of the microchannels has demonstrated excellent control over the final density, effective thermal conductivity, and mechanical performance of the resulting channeled material (with the tensile, flexure, compression, and impact properties having been evaluated to date). Additionally, control over the channel diameter allows for enhanced thermal insulation performance, as gas-phase conduction within the channels becomes limited as the channel diameters become comparable to (or smaller than) the molecular mean free path of the gas species contained within the channel network structure. Final channeled specimens exhibiting the effects imparted by the engineered channel architecture were also reinforced with low areal density fiber veils to improve the strength, stiffness, and impact performance without compromising the thermal insulation capabilities of the composite part. Rather than adhesively bonding facesheets to a microchannel core, the novel manufacturing methodology allows for integrative fabrication of the sandwich composites using both the traditional facesheet approach as well as an innovative multi-layered hybrid sandwich composite suitable for highly engineered design strategies. Despite the fact that these sandwich composites utilize the lightest carbon and aramid fiber veils commercially available, the reinforced channeled architectures demonstrate the robust mechanical properties, low bulk density, and excellent thermal insulation performance necessary for structural, thermally insulating composite materials for use in transportation and construction applications.
Improved Manufacturing of High-Performance Composites by Frontal Polymerization
Current manufacturing methods for high-performance fiber-reinforced polymer composites (FRPCs) are time and energy intensive, and their cost and complexity often act as a deterrent to wider the adoption of these materials. Frontal polymerization (FP) has recently been demonstrated as an energy-efficient, out-of-autoclave technique for the manufacture of thermosetting FRPCs. In this process, polymerization is achieved through a self-propagating exothermic reaction wave, triggered by an external thermal stimulus, that converts the liquid monomer to a fully cured polymer. Here, we demonstrate rapid manufacture of woven carbon fiber-reinforced composites with fiber volume fractions, up to up to 66%, a 30% increase over prior FP-based processing methods, using a combination of hot-pressing techniques, vacuum-assisted resin transfer molding, and FP. Higher values of the polymerization rate, maximum front temperature, degree of cure, glass transition temperature, and fiber volume fraction of these FRPCs are achieved by using different catalyst loadings and thermal triggering mechanisms. A systematic study reveals the effect of varying the catalyst loading on the mechanical and thermomechanical properties of the resulting FRPCs.
Effect of Lightning Strike on the Compressive Strength of Carbon Fiber Reinforced Polymers—A Finite Element Study
The use of polymer composites as structural materials has increased drastically in the recent past. While they possess qualities like high strength to weight ratio and corrosion resistance, the poor electrical and thermal properties make them vulnerable to damage due to lightning strikes. When struck by lightning, the electrical current takes more time to travel through polymers (matrix) as they are poor electrical conductors and causes heating near the strike area (resistive heating). The temperature gradient created by resistive heating leads to a change in material properties and results in the stiffness reduction of the structure. The carbon fiber reinforced polymers (CFRP), a widely used material in aircraft and wind turbine industries is weak in compression. Under pure compressive loading, the fiber-reinforced polymers (FRP) undergo localized buckling (kinking), due to the fiber misalignment (waviness) introduced during manufacturing. Kinking is identified as one of the failure modes in FRPs. After the lightning strike, kinking in FRP would be accelerated as the resistive heat generated causes thermal strains in addition to mechanical strains in the polymer matrix. This in turn leads to a drastic reduction in the compressive strength of the material. In the current study, a finite element (FE) analysis was conducted to obtain the compressive response of CFRP subjected to different current intensities using the commercial software ABAQUS.
Manufacturing Simulation of Carbon Fiber Reinforced Composites for Evaluation of Residual Stress and Strains
In this research, we propose a computational method to predict the residual stresses and strains of carbon fiber composite by considering the curing kinetics, chemical shrinkage, and thermal expansion. During the curing process of the composites manufacturing, the chemical shrinkage, thermal expansion, and resin viscoelastic behavior lead the residual strains and stresses. Predicting the residual stresses and strains is important to evaluate the deformation and mechanical properties of composite materials. However, this is expensive and time-consuming. On the other hand, the computational method can be used to replace the actual experiments and save costs. In this work, material curing kinetics are computed according to a given cure cycle temperature profile. After computing the degree of cure, the mechanical properties of the composite material are estimated. From the given values, the stress of the composites is evaluated with stress-strain relation. The distortion predicted by the proposed computational model is then verified with the experimental model manufactured with RTM (Resin Transfer Molding) equipment.
A Computationally Efficient Thermomechanical Model for the In-Situ Polymerization of a Methyl Methacrylate-Based Resin in a Thick Glass Fiber Laminate
A computationally efficient thermochemical model is used to predict thermal runaway and the occurrence of monomer boiling during the in-situ polymerization of a methyl methacrylate (MMA)-based resin within a thick glass fiber layup. The framework couples the auto-accelerating exothermic free-radical bulk polymerization of MMA with the transfer of heat within the system and to the environment. Three approaches are considered for the kinetic treatment of the auto-acceleration: (1) an empirical model, (2) a semi-empirical model and (3) an analytical model. Predictions made via the three kinetic models are compared with experimental data measured during infusion tests with different processing conditions. Processing diagrams are constructed to reveal manufacturing regimes of interest for the production of thick fiber-reinforced methacrylic composites free of voids due to monomer boiling.
Minimizing the thermoplastics weight is of paramount importance in shipping, aviation, industrial components, and submarine vehicles. Strain rates vary in the in-field loading scenarios and hence needs to be investigated as a function of filler content. In the present work, lightweight feedstock filaments are developed using glass microballoons (GMB) as filler and High-density polyethylene (HDPE) as the matrix. The blend is prepared by varying GMB content between 20 and 60 volume %. Filaments are fed to a commercially available 3D printer and subsequently printed samples are tested under compression (0.001 and 0.1 S-1 strain rates). Printed foams manifested higher moduli at higher strain rate as compared to neat HDPE. With increasing GMB content and strain rate, yield strength decreases. Energy absorption for foams increases with a higher strain rates. Specific compressive modulus increases while comparable strength is observed in prints as compared to HDPE.
Measurement of Constitutive Properties of Additively Manufactured Epoxy Composites for Application to Sandwich Structures
Additive manufacturing technologies are beginning to find a foothold in industrial applications which call for complex geometries and low production volumes. In particular, the aerospace industry is targeting additive manufacturing to enable increased design freedom to reduce weight without sacrificing performance. Most industrial applications use either photopolymers or thermoplastics which can be filled with discontinuous reinforcement. Thermosetting polymers which are preferred by aerospace are not widely used for additive manufacturing on an industrial scale because there is a lack of available material systems and material models for design and prediction of part performance. To address these needs, an epoxy ink loaded with 5.5 vol% chopped carbon fiber is printed and select mechanical properties are measured. Mechanical tests include: tension, flexure, compression, and in-plane shear. Properties generated here are being used to populate a material model for predicting performance of hybrid sandwich composites with composite face sheets and printed cores. A comparison is made between two printed sandwich core geometries, rectangular and honeycomb, with rectangular cores achieving at least 2.6x greater load at break.
Prediction of Crack Propagation Along Non-Uniform Raster Patterns in AM Composites
Chopped fibers are commonly incorporated to composites manufactured by Fused Filament Fabrication to increase the mechanical properties in the primary raster direction. However, these composites are notoriously brittle in the transverse direction. In an attempt to improve the fracture toughness in the transverse direction, we propose to modify the raster path with a zig-zag architecture. Using both experiments and discrete damage modeling, we show that modified raster patterns successfully deflect cracks. The fracture mechanism transitions from interfilament delamination at low raster deflection angles, to transfilament fracture at larger raster deflection angles. Conceptually, this approach can be used to enhance fracture toughness, or steer cracks away from critical components embedded in FFF composites
Hot Isostatic Pressing to Enhance Inter-Laminar Tensile Strength in Additively Manufactured Carbon Fiber-PEEK Parts
Additive manufacturing (AM) of composites is a rapidly expanding technology with an increasing focus on printing end use parts rather than prototyping. Fused filament fabrication (FFF) is relatively inexpensive and fast compared to other AM technologies. The resulting FFF parts, however, suffer low interlaminar mechanical properties due to poor layer-to-layer adhesion and voids. A possible solution to this would be post processing using hot isostatic pressing (HIP). HIP utilizes high temperatures and pressures to consolidate parts in turn reducing void content, enhancing inter-layer adhesion, and increasing the crystallinity of the parts. This paper seeks to investigate the effects of HIP on structure and interlaminar strength of short carbon fiber reinforced polyetherketoneketone (PEKK) parts manufactured via FFF. Coupons were printed vertically (normal to the print bed and along the Z-direction) and post processed for one hour in an in-house HIP device. The HIP pressure was set to 0, 20, and 200 psig and a constant temperature of 165 °C (slightly above its glass transition temperature) was used. HIP treated samples were characterized via tensile tests to evaluate their tensile strength and modulus. Additionally, the samples were analyzed using X-ray micro computed tomography (μCT) device to assess their void content before and after the HIP processing. Structure, morphology, and properties for all tested samples were compared and correlated to the HIP treatment conditions. It was concluded that HIP can effectively increase the inter-layer mechanical properties of FFF parts by up to 50% in strength and 30% in modulus, rendering them more suitable for end use applications. Based on the new understanding of failure resulted from this study, post-treatment strategies to further increase the Z tensile properties in FFF are suggested.
3D Printing of Kenaf Fiber Reinforced Thermoplastic Polymer Composites
Additive manufacturing processes such as fused filament fabrication (FFF) has recently gained interest within many manufacturing industries; however, there is a limitation on existing advanced composite materials availability. The innovative combination of 3D printing with sustainable materials such as natural fibers has the potential to transform the way many products are designed and fabricated. The current work understands how the addition of kenaf fibers to thermoplastic copolyester elastomer affects the characteristics of a material associated with its suitability for 3D printing (TPC). Additionally, to enhance fiber-polymer bonding, Kenaf fiber treated with acid and alkaline solutions, along with untreated fibers, were investigated as reinforcing fillers within a TPC matrix. Three incremental levels of fiber loading (5, 10, and 15 vol.%) of the composite materials were investigated. Tensile test specimens made from the composites were created using an FFF printer. Our study findings indicate that both tensile strength and elongation decreased with increasing fiber loading for 3D printed. However, yield strength and stiffness of the composites increased with fiber loading as much as two folds for 15% fiber loading.
High Speed Puncture Properties of Additively Manufactured ULTEMTM 9085 Exposed to Hot/Wet Conditions
High-speed puncture testing (ASTM D3763 – 10) was conducted on ULTEMTM 9085 sample disks created on a Stratasys FORTUS 900mc fused deposition modeling (FDM) machine. Samples of various thicknesses, print direction, and subject to various environmental conditioning, were tested. Statistically significant differences in peak load and absorbed energy were noted among the samples. Finite-element simulations of the impact tests were performed in LS-DYNA using an elastic-plastic constitutive model, calibrated to quasi-static tensile data collected on samples exposed to the same environmental conditions as the puncture tests. The numerical simulations showed qualitative agreement with experimental load-displacement curves up to peak load. Our results suggest that continuum constitutive models and commercial finiteelement analysis tools, calibrated to limited but physically congruent mechanical test data, show promise for first-order predictions of the puncture performance of as-built FDM components.
Digital Design and Manufacturing with Thermomechanical Process Simulation for Fused Filament Fabrication 3D Printing
Recent demonstrations with fused filament fabrication (FFF) 3D printing has shown to produce protypes as well as production components. Additionally, due to the FFF process platforms being low-cost and readily available there has been a high-demand to produce on-demand parts for various applications in automotive, in-space manufacturing and electronic industries. However, current limitations such as limited availability of advanced composites materials, and guidelines for designfor- manufacturing make the process prone to trial-and-error experiments both at the materials development, product design and manufacturing stage. In this work, new thermomechanical process simulations platform, Digimat-AM has been evaluated to address and demonstrate digital design and manufacturing of FFF process by performing simulation and experiments. With the use of Acrylonitrile butadiene (ABS) material as a basis, a L9 Taguchi design-of-experiment (DOE) was setup by varying key process input parameters for FFF 3D printing such as layer thickness, melt temperature and extrusion multiplier were varied for three levels. A total of 9 DOE simulations and experiments were performed to compare part properties such as dimensions, warpage, and print time were analysed. Additionally, ANOVA analysis was performed to identify the optimum and the worst conditions for printing and correlate them with their effect on mechanical properties of the printed samples. Furthermore, from the simulation results, a reverse warpage geometry, 3D model was generated that factors for part warpage, shrinkage, or other defects to enable 3D printing parts to design dimensions. Subsequently, using the generated reversed warpage geometry was used to perform 3D printed experiments and analysed for part dimensions and defects. As a case study, a functional prototype [Two different geometries] was designed and simulated on Digimat-AM and using the above guide, 3D printing was performed to obtain part to specific dimensions.
Formation of Letters a, b, c Using 4D Printing of Composites
This paper presents the procedure for the manufacturing of composite structures with complex geometries using the method of 4D printing of composites (4DPC). In 4DPC, the complex structures are made without the use of complex molds. Flat layers of composites are deposited on a flat mold. Utilizing the anisotropy of the lay up sequence in the laminates, when these laminates are cured and cooled to room temperature, the flat laminates become curved. In this paper, the complex configurations of the letters a, b, c are made using the method of 4DPC. The lay up sequence for each of the letters along with their final configurations are presented.
Twisted Pathways Towards Tougher Interlaminar Regions in Laminated Composites
In this paper, we build upon a previously introduced interface enhancement fabrication method that has shown to improve interlaminar fracture toughness of laminated polymeric composites by 25% in mode-I and by 76% in mode-II. This method utilizes filament extrusion 3D printers to precisely deposit molted polymer directly onto carbon fiber prepreg. Upon bonding with the prepreg, these interlaminar polymer reinforcements act as barriers to a planar crack, causing the crack to tilt – requiring more energy, and manifesting as an increased fracture toughness. In the current study, we optimize the pattern of the printed reinforcement to induce crack twisting alongside crack tilting to further increase the interlaminar fracture toughness. In Mode-II, we see a further improvement of ~17% while still using the same amount of polymeric material deposited in the interlaminar region – that is an overall improvement of ~95% compared to pristine samples. Using microscopy and microcomputed tomography ( ðœ‡ ð¶ ð‘‡ô€ˆŒ, we observe the wavy crack path during delamination.
Additive Manufacturing of Controlled Porous Elastomeric Nanocomposites for Enhanced Sensing Function
Elastomeric sensors have a wide range of applications in fields such as structural health monitoring, robotics, and biomedical industries. The additive manufacturing of these sensors, achieved using Direct Ink Writing (DIW), has provided numerous advantages, including increased sensitivity and the fabrication of complex geometry. However, bulk material sensors manufactured additively or using conventional methods, display a more significant effect of hysteresis, especially at strains higher than 10%, which limits their sensitivity. They also show a higher level of material relaxation whereby the resistance change within the material decreases in cyclic loading of the same strain and strain rate. Sensors with significant amounts of porosity have decreased hysteresis and high sensitivity at the lower strain. Porosity can be introduced in the form of varying infill densities and patterns made possible by DIW, which reduces these bulk material effects. In this paper, a lattice structure with four infill densities are investigated, and the samples are 3D printed using a grid infill pattern. The fabricated samples are characterized using a scanning electron microscope (SEM) to validate the microstructural features and layer bonding. Each sensor’s pressure-sensing capability is investigated using cyclic compression loading at various maximum strains. Sensing experiments show an increase in strain sensitivity with the introduction in porosity, compared to bulk samples of the same material and geometry. It is found that introducing porosity using DIW is a sensible strategy to improve the piezoresistive performance of nanocomposites and to allow for the tunability of sensing capacity in piezoresistive strain sensors.
Development of Virtual Process Simulation and Validation for a Carbon Fiber Reinforcement Panel
Carbon fiber reinforced polymer composites (CFRP) are increasingly utilized in aerospace and automotive industries for lightweight structural applications due to their excellent properties. However, challenges remain in the molding of structural composite parts having a high fiber content with a short cycle time. The permeability of the fabric reinforcement decreases exponentially as the fiber content increases, leading to a long filling time when low pressure injection is used. This, in turn, causes resin gelation during the filling stage resulting in partially filled parts due to the long filling time. High Pressure Resin Transfer Molding (HP-RTM) is a recent variant of Resin Transfer Molding (RTM) technology that enables a high resin injection rate and thus a short cycle time when new fast curing resins are used. In this work, a carbon fiber reinforcement panel of an all-carbon fiber floor assembly is successfully designed and fabricated using HP-RTM. To better understand the physics of the resin flowing through a porous fiber bed, which is the fundamental aspect of the HP-RTM process, this paper discusses the Finite Element Analysis (FEA) development used to predict the location of the resin flow front, the in-mold resin pressure profile, and the degree of cure of the resin with selected continuous carbon fiber fabrics. The developed FEA framework is used to guide the tool design and determine the locations of the injection gates and vacuum ports. We have developed a semi-automated molding procedure by synchronizing the press and HP-RTM metering control system to enable the stepwise vacuum and injection strategy while a predetermined tonnage is maintained. The reinforcement tool is equipped with in-mold pressure sensors and dielectric cure sensors for data acquisition during molding. The correlation between the experimental data and simulation predictions shows good agreement regarding the resin filling time and the in-mold pressures during the resin injection phase. An overall cycle time of 3 minutes is successfully achieved to mold the reinforcement component.
Reverse Engineering of 3D Printed Composite Part by Toolpath Reconstruction
Development of composite material parts requires significant research and development effort. The fiber length, volume fraction and direction are important parameters in determining the properties of the part and are determined by extensive theoretical analysis, finite element analysis and experimental studies. Composite parts are now used in aircraft, space structures, and marine vessels extensively. Additive manufacturing (AM) methods are increasingly used for printing composite material parts because of their low production volume and complex structure, which is difficult to manufacture by other methods. Non-destructive imaging methods are widely used for quality assessment, for example micro-CT scan is used for porosity analysis in composite parts. Advancements in 3D scanning and imaging technology have raised a significant concern in reverse engineering of parts made by AM, which may result in counterfeiting and unauthorized production of high-quality parts. This work is focused on using imaging methods and machine learning (ML) to identify fiber direction in various sections of the composite for possible reverse engineering. Here not only the geometry is captured but also the tool path of 3D printing is captured in various sections of the microstructure. A dimensional accuracy with only 0.33% difference is achieved for the reverse engineered model.
Numerical Simulation of Resin-Fiber Interactions for Advanced Composites Manufacturing
Advanced composites are key enablers to initiatives such as aircraft/automobile light-weighting and construction of Out of Autoclave (OOA) and reused/recycled composite sports equipment [1-13]. The integrity of composite materials is only as good as the manufacturing process used to create it, thus a detailed understanding of that process and the imperfections it entails is paramount [14-15]. Manufacturing of many ubiquitous composites involves the wetting of fibers (such as glass fiber, or carbon fiber) with liquid resin. The wetting efficiency of the resin prior to hardening/curing directly affects the resulting material’s strength and integrity. In this study, we consider a canonical setup in which an isolated resin droplet interacts with a bed of fibers below. We use computational fluid dynamics (CFD) to gain detailed insight into the wetting process itself, making use of ANSYS CFD tools. Within the framework of ANSYS Fluent, we use the volume-of-fluid (VOF) method to capture the dynamic interface between the resin, air, and fibers. Fiber wetting processes involve drop-to-fiber diameter ratios in the hundreds to thousands, which can make the simulation approach computationally expensive. In this work, we use two-dimensional simulations (e.g., see Figure 1) to characterize the drop-fiber interaction and qualitatively verify the governing nondimensional parameters revealed through dimensional analysis. Three-dimensional simulations are then used to assess resin spreading and penetration as a function of fiber orientation, comparing results to experimental data obtained at the University of Southern California.
Transverse Failure Behavior of Continuous Fiber Reinforced Composites
Composite material reveals exceptional high stiffness, strength and stiffness-toweight ratio values which properties can advantageously be used in many demanding engineering applications. However, due to the processing variations and the resulting microstructures (including defects) the macroscopic material properties reveal a high data scatter. A stochastic methodology was described by a research group also with our contribution to predict the properties of UD composites. The present paper is a simplification and includes further development of this methodology for uniaxial thermoset and thermoplastic fiber reinforced composites. Stochastic microcells for UD laminates were generated using a software tool developed in above project. The matrix material properties correspond to a specific epoxy resin typically used for high performance composites and to a polycarbonate recently used in thermoplastic UD tapes. The basic material data were determined in terms of elastic modulus, tensile strength, failure strain and fracture toughness values. The microcell model consists of the typical failure modes of a 2D cell, matrix failure (elastic-plastic) of the matrices and cohesive strength for the fiber matrix interface. To simplify the continuous distribution functions, discrete values over a wide range were selected for all of the parameters used in the models and simulations with these discrete values were carried out. The cells were exposed to tensile and compressive monotonic loading transverse to the fiber direction. The macroscopic transverse strength (Yt, Yc and S) values over a wide material parameter range were predicted for conventional composite failure models. The predicted failure values were compared with corresponding experimental values which were measured using UD laminates in transverse to fiber direction. The sufficient amount of the specimen tests enabled a statistical analysis of the experimental data.
A Constitutive Model for Materials with Transversely Isotropic Strain Locking Microstructures
Strain locking materials have a limit to the degree to which they can be stretched along one or more axes. The limiting strain behavior is due to stiff microstructure components that reorient along the direction of the applied load as stretch increases. Both natural and manmade materials can exhibit such a response when initially wavy fibers or other corrugated structures gradually straighten and limit the extensibility of the material. Two new constitutive models are developed which describe materials with strain locking structures that are oriented along a particular axis. The models assume the microstructure is composed of linear elastic material with embedded zigzag shaped, series of structures oriented along a preferential axis which produce the locking behavior. The response is governed by a strain energy density function which is partitioned into separate portions that represent the stored energy within the fibers and the matrix respectively. A new, nonlinear form for the strain energy density function of strain locking fibers is derived based on the change in angle of the fiber’s vertices. Examples of the mechanical response are demonstrated for a matrix material defined by a linear elastic model. The model parameters are fit to tensile stress versus strain data generated from Finite Element Analysis of a strain locking composite RVE as well as experimental data from a strain locking composite thin-film material. Correlation with the experimental data shows that the proposed constitutive model accurately fits the initial tangent modulus and ultimate strain locking limit well.
Optical Characterization of Nanosilica-filled Bisphenol-F Epoxy and Carbon Fiber Composites
In this investigation, we examined the effect of curing agents and carbon fiber on the color of epoxy-based composites with varying percentages of nanosilica (NS) as filler. Composites with different concentrations of NS were prepared using a derivative of bisphenol F (bisF) epoxy, cured with diethyl-toluene-diamine (DETDA) or 4,4’- Diaminodiphenylsulfone (DDS), along with carbon fiber composites with bisF and DETDA. The variations of color across samples were correlated to the reflectance in the UV-Vis regimes. We found that curing of samples by DETDA, rather than DDS, leads to higher reflectance in the UV-Vis regimes and almost no effect in the NIR regime. Increments in reflectance within the visible regime are proportional to the NS concentration, whereas such proportionality is not observed in the NIR regime. Reflectance up to 45% is observed initially on addition of 15 wt% NS in both epoxies in NIR spectrum, but further NS addition does not increase reflectance. While the addition of carbon fibers increases mechanical properties, we found their addition makes the composites vulnerable to photodegradation. While the addition of carbon fibers increases mechanical properties, we found their addition makes the composites vulnerable to photodegradation.
Interlaminar Shear Strength of Carbon/Epoxy Composite with Aligned Carbon Nanotube Yarn Interlayers
Carbon nanotubes can provide significant vibration damping to carbon/epoxy composites, which is a benefit for high vibration applications such as rotorcraft. One means of maximizing the volumetric concentration of CNTs, and thus damping, in composite laminates is to concentrate the CNTs in the interlayer regions rather than disperse them throughout the matrix. Also, aligning the CNTs along the principal strain direction maximizes damping. However, there is concern that the weak CNT/epoxy bond will compromise the interlaminar shear strength (ILSS) of the laminate. The current investigation aims to evaluate the ILSS of unidirectional carbon/epoxy with an interlayer consisting of CNT yarns. The ASTM D2344 short beam strength (SBS) test is used to evaluate the ILSS of 0-deg. unidirectional carbon/epoxy with a single layer of aligned CNTs at the midplane, where failure occurs predominantly due to the applied shear stress. The main variables in the tests are the non-ionic surfactant and the orientation of the CNTs (parallel or perpendicular to the shear direction). The SBS results indicate negligible reduction in ILSS due to the addition of just surfactant to the composite, although the glass transition temperature of the composite is reduced by 10°C. Upon the addition of surfactant-treated CNTs parallel to the shear direction, the ILSS reduction relative to the baseline material is small (4%). When the surfactanttreated CNTs are oriented perpendicular to the shear direction, the reduction in ILSS relative to the baseline is also small (8%). It is concluded that adding concentrated surfactant-treated CNT interlayers to carbon/epoxy laminates has a small adverse effect on ILSS and glass transition temperature.
Effects of Periodic Stitching Patterns on the Strain Distribution in Polymer Composites
Traditional laminated composite structures suffer from low interlaminar strengths, which may result in delaminations. Stitching through the thickness has demonstrated to reduce the initiation and growth of delamination damage. This study investigates the effects of periodic stitching patterns on the strain distributions in three-dimensional (3D) stitched carbon composite structures. Unstitched and stitched specimens with two different periodic stitching patterns (0o and 90o) were fabricated and tested under uniaxial loading. The surface strain fields were obtained using digital image correlation (DIC). Noticeable differences were seen in the strain distribution of these periodic stitching patterns. The global stiffness of the laminates and the local (near the vicinity of stitching) tensile modulus of elasticity were altered due to stitching. Therefore, periodic stitching gives rise to localized regions of tailored elastic properties.
Fracture Toughness of Stitched CFRP Composites Using Optical Fiber Strains
The use of lightweight polymer matrix composite materials in primary load-bearing applications has increased in the aerospace industry due to their high strength and stiffness. However, delamination caused by low interlaminar fracture resistance can cause stiffness degradation and affect the integrity of the structural part. Throughthickness stitching is used to improve the delamination resistance by providing translaminar reinforcement. In order to characterize fracture behavior in composite laminates, fracture energy is determined by performing double cantilever (DCB) tests. Stitched DCB specimens with a quasi-isotropic layup configuration were fabricated using vacuum-assisted resin transfer molding. Dry preforms were stitched using a robotic single-sided stitching technique. To obtain the internal strain data and internal crack growth, an optical fiber was embedded at the midplane of the specimens. The apparent fracture energy was estimated using the modified compliance calibration method. The strain data obtained from the optical fibers enabled real-time crack monitoring of the DCB specimens.
Experimental and Numerical Investigation of Large Scale Effect on Buckling and Post-Buckling Behavior of Tubular Structures
Buckling and post-buckling behavior of tubular structures can essentially be affected by the component geometry such as length, diameter, and wall thickness. In this paper, an experimental and numerical study of aluminum and multilayer carbon fiber reinforced plastic (CFRP) tubes under compressive load is discussed, with the aim to extend the knowledge towards extremely large tubular structures. A pin-ended fixture is designed to examine the influence of different support conditions. Experimental results of aluminum tubes show global failure in which critical forces are validated by the Johnson-Euler buckling theory. It also demonstrates that the critical force of a long tube can be the same as a short tube with different ending conditions. However, the tests of CFRP tubes demonstrate more material failure mechanisms and local buckling failure phenomenon. Buckling and post-buckling behavior of CFRP is further investigated by using finite element analysis. A three-dimensional damage model based on the Hashin’s failure initiation criteria and cohesive zone method is developed to analyze tensile and compressive damage on matrix and fiber. The combination of experimental and numerical results aims to identify the scale effect on failure mechanisms, failure location, buckling mode, and compressive response. Results show that the stiffness of shorter tube drops significantly due to inplane shear damage initiation and development. For longer tubes, the stiffness remains the same until the occurrence of local buckling failure, owing to sudden release of strain energy upon arrival of critical force. The main idea in this work is to predict the buckling model and behavior in extremely large aerospace components. Since it would be impractical to test large structures due to the equipment limitation and high cost, the prediction models used in this study would be beneficial for practical designs and applications
In-situ Healing of Mode-I Fatigue Crack in CFRP Composite Using Thermoplastic Healant
The healing performance of Mode-I interlaminar fracture of a carbon fiber-reinforced polymer (CFRP) composites subjected to fatigue loading is investigated in this research. Laminated composites are highly susceptible to delamination, and delamination due to fatigue loading is one of the most critical damage modes in composite structures that may lead to a catastrophic failure. Hence, it is paramount to investigate and quantify the delamination crack growth behavior due to fatigue loading and explore methods to heal the delamination. To this end, double cantilever beam (DCB) specimens of a carbon fiber-reinforced polymer (CFRP) composite containing thermoplastic healants were manufactured, and the Mode-I fatigue delamination experiments were carried out for virgin and five healing cycles. In this study, in-situ healing performance of Mode-I interlaminar fracture of CFRP composites subjected to fatigue loading up to seven healing cycles was investigated. The main purpose of using thermoplastic healants (polycaprolactone (PCL) and shape memory polymer (SMP)) was to heal the cracks formed during fatigue loading as well as to regain its load-carrying capacity. To establish proof of concept, the healing was initially achieved with the help of a convection oven where the heating is provided to close the cracks using thermoplastic healants. From experimental data for virgin and healed specimens, the Paris law parameters were extracted, and the results obtained from different specimens were found to be repeatable upon the comparison. Furthermore, the slope parameter of the Paris law for the healed specimens was found to be almost 75% ~ 86% of the virgin specimen, thereby indicating slower crack growth in the healed specimens under fatigue loading. Subsequently, in-situ delamination healing was performed by activating macro fiber composite (MFC) actuators attached to the DCB specimen after 5000 cycles of loading. For the fifth, sixth, and seventh healing cycle, the crack closure observed after insitu healing was significant with 10% ~ 12% reduction in crack growth after 21000 cycles with respect to the virgin cycle. These findings are envisioned to be very helpful in extending the service life of self-healing composite smart-structures and thereby provide huge repair cost savings.
Polymers with rubber-like elasticity, also known as elastomers, are among promising candidates in ballistic protection products (armors) due to their low weight and high energy dissipation characteristics. Dynamic mechanical properties of elastomers, including their damping factor, can be enhanced by incorporations of fillers which are also expected to improve the ballistic protection capability of armor systems. In this study, dynamic response of such elastomers filled with micrometer-size carbon fibers at various high rates and temperatures above their glass transition is directly assessed using split-Hokinson pressure bar (SHPB). The dynamic mechanical analysis (DMA) test results are then compared with these direct measurements using timetemperature superposition (TTS) principle to construct frequency domain mastercurves, yielding stiffness and loss properties over a wide range of frequencies. It is observed that the incorporation of fillers within an elastomeric matrix can significantly increase its strength and stiffness while maintaining the loss properties. The effect of fillers volume ratio is further studied. It is shown that, although increasing the volume ratio can improve stiffness, the elastomer composite with excess amount of fillers will have higher induced shear stress between the matrix and fillers. The higher volume fraction of fillers can also increase the prospects of introducing more defects which at the end can negatively affect the ultimate toughness of the composite. This work may pave the way for designing more effective ballistic protection products at various impact speeds.
Impact of Ultrasonication on Carbon Nanotube Demixing and Damage in Polymer Nanocomposites
The processing technique can significantly impact how nanoparticles are distributed and dispersed within a polymeric matrix. Homogeneity in spatial distribution and effective nanoparticle dispersion are necessary to achieve optimal polymer nanocomposite properties and can be a processing challenge. Here in particular we study the impact of ultrasonication on the dispersion and distribution of carbon nanotubes (CNTs) in the polymer nanocomposite. Specifically, we seek to better understand the interplay between nanotube demixing and damage for excessive sonication times. Here polymer nanocomposite samples of poly(caprolactone) (PCL) were mixed with CNTs using a ultrasonication process where the samples are subjected to different durations of CNT/solvent sonication. In addition to common dispersion characterization techniques, an analysis of mixing indices and the shear-induced crystallization behavior of the nanocomposites were found to be particularly insightful when used to study the effects of sonication time on nanoparticle dispersion. Findings based on TEM evidence of CNT damage, optical image analysis, and the shear-induced crystallization behavior of the samples suggest that mechanisms of demixing and damage of the CNTs co-exist within the processed nanocomposite and that both contribute to the reduction in effective properties for excessive sonication times.
Novel Homogenized 3D Brick Element for Multiscale Modeling Using Mechanics of Structure Genome
Traditional multiscale analysis homogenizes a heterogeneous body to be an equivalent homogeneous body with the effective material stiffness matrix or compliance matrix computed from a representative volume element (RVE). Traditional multiscale analysis assumes that there is a clear scale separation between the microstructure and the macroscopic structure so that the microstructure can be considered as a point in the macroscopic analysis. This is only true if the microstructure repeats many times around one point within the macroscopic structure. However, in most real applications, such repeated microstructure does not exist. In this paper, we will develop a novel homogenized 3D element using the mechanics of structure genome (MSG) by homogenizing a heterogeneous body discretized by many elements into a single 8-node or 20-node brick element. The theory would generate an effective element stiffness matrix, which can be used as input for a 3D brick element developed using the Abaqus user element subroutine (UEL). After computing the macroscopic behavior using the UEL elements, the local displacements, stresses, and strains can be computed through a dehomogenization process based on MSG. To verify the developed theory, we created three different examples to demonstrate its application and accuracy in comparison to other alternative approaches.
Effect of Moisture on the Tensile Properties of Composites with Bio-based Fibers and Matrix
Fibers and matrices made in full or part with plant-based (bio-based) materials are attracting increased attention in the composites industry on account of their potential for environmental sustainability. However, many bio-based constituents are known to be susceptible to moisture absorption and associated degradation. The objectives of this investigation are to evaluate the quasi-static tensile behavior of composites made with different combinations of jute and E-glass fibers and biobased and synthetic (fossil-fuel based) epoxy matrices in the as-received condition and after a 24-hour soak in water at 66°C. The composites were manufactured using vacuum-assisted resin transfer molding. Fiber volume contents were determined for the composites. Tensile strength and modulus were measured on as-received and water-soaked composites. After water exposure, the jute fiber composites suffered a large reductions in modulus with either type of matrix and a large reduction in strength with the bio-based matrix but not the synthetic matrix. The E-glass composite with the synthetic matrix, which was only tested in the as-received condition, had much higher modulus and strength in comparison to the jute fiber composites.
Design and Evaluation of Load Path Tailored Composite Panels
The aim of present study is to establish the optimization design approach for light weight composites aircraft structure. A composite panel with a large cut-out representing a part of lower wing panel is optimally designed with the curved fiber orientation based on principal load path. The optimum curved fiber orientation is directly defined by the principal stress direction derived from finite element analysis result. Only the fiber orientation in 0 degree layers are steered based on principal load path. Three different types of the optimum panel with different staggering strategies are precisely manufactured by Automated Fiber Placement (AFP) with tow-steering layup technique. The optimum panels are evaluated by testing under tensile load. The results show that the initial failure load of the optimum panels with curved fiber orientation is 6% higher than that of the conventional panel with straight fiber orientation with the same weight, and the panel stiffness of the optimum panel is 3% higher than that of the conventional panel. The maximum strain level around the cut-out of the optimum panel is 8% smaller than that of the conventional panel. These results indicate that the optimum panel is capable of sustaining larger load than the conventional panel, and imply the potential of structural weight reduction for load path tailored composite panel. Those improvements in the testing correspond well with the numerical prediction which is used for the panel design.
Stiffness Degradation and the Fatigue Behavior of Low-flow Carbon Fiber Sheet Molding Compounds
Carbon Fiber Sheet Molding Compounds (CF-SMC) combine the properties of high-performance prepregs with the cheaper and easier manufacturing possibilities of SMCs. Understanding the fatigue behavior of these composite materials is essential for demanding engineering applications where components are exposed to cyclic loading conditions. This contribution investigates the fatigue behavior of CF-SMC based on discontinuous prepreg platelets with a random orientation distribution. CFSMC plates for specimen preparation were compression molded in a low-flow setup. Fatigue tests were performed under uniaxial cyclic tension-tension (R = 0.1), compression-compression (R = 10) and tension-compression (R = -1) loading conditions. The stiffness degradation was significant and fundamentally different in tension compared to compression fatigue. Stiffness reduction happened sooner in the early stages of fatigue life for compression-compression and tension-compression fatigue loading than for tension-tension. In-situ passive thermography measurements for the tension-tension fatigue tests revealed diffuse damage as well as localized major damage sites. The final failure site could be correlated with the first visible appearance of a localized temperature increase by passive thermography. These findings suggest that the weakest link (the least favorable local mesoscale morphology) defines the fatigue strength of CF-SMC. A damage accumulation time was introduced which could potentially be a useful tool in terms of fatigue life prediction and structural health monitoring of CF-SMC structures.
Modified Lap Shear Test for Automated Fiber Placement (AFP) of Steered Thermoplastic Composite Tape
Manufacturing of Variable Angle Tow (VAT) laminates by in-situ consolidation of thermoplastic composite tapes using Automated Fiber Placement (AFP) or Automated Tape Laying (ATL) techniques has gained recent popularity due to its ability to efficiently manufacture composite laminates without the need for secondary processes like resin infusion or curing in an autoclave. However, currently there is no standard method for testing the bond strength of the steered in-situ consolidated composite tapes to the substrate. This study proposes a specially devised lap shear method to test the bond strength of a steered and in-situ consolidated tape. Sample preparation as well as testing procedure are described in detail and experimental results are presented. Samples for this study were manufactured using AFP process with Hot Gas Torch (HGT) as the heating system available at Concordia Center for Composites (CONCOM) and were tested using universal testing machine. To show the repeatability of the proposed lap shear test, a number of samples with different steering radii were tested and statistical data are presented.
Full-Field Experimental Analysis of Sandwich Plates/Beams Under Bending
A composite sandwich structure made with woven face sheets and a relatively compliant core was loaded in 3-point bending. An image was captured of the specimen before and after deformation, which was then analyzed with a Digital Image Correlation (DIC) program. The displacements and strain fields were obtained for the specimen. The deflection of the beam was compared with 5 theoretical model and 1 numerical solution for a sandwich beam experiencing loading in a 3-point bending configuration.
Novel Techniques for Investigating Shear of PMCs
Composite materials continue to see increasing usage in a variety of industries due to their high strength to weight ratio. To incorporate composites in aerospace and marine applications, certification and damage tolerance assessment must be performed. Damage evolution, specifically under shear stresses, can be particularly challenging to predict. Shear failure in composite structures is difficult to characterize experimentally. Developing physics-based theories for computational modeling composites is crucial and must be substantiated with experimental evidence. Local tri-axial stress states must be considered for a valid shear experiment. For a near pure shear test, the hydrostatic pressure must be significantly less than the equivalent stress at the fracture location [1]. A novel technique to investigate the shear failure of PMCs, inspired by the isotropic butterfly notch specimen, is proposed. The novel technique uses uniaxial loading in compression or tension; however, a specific region of interest locally fails under shear load conditions. Several panels of IM7/977-3 with various stacking sequences were manufactured using traditional autoclave processing of prepreg. Specimens were fabricated using a waterjet. Tests were conducted in tension and compression while non-contact surface strain measurements were recorded utilizing digital image correlation (DIC). Post failure, specimens were characterized using micro computed X-ray tomography (μCT) to analyze the failure mechanisms. Additional work used discrete damage modeling techniques to simulate the load configurations help validate a newly implemented shear failure criteria. Additional comparisons to existing failure criterions, such as LARCO4, were made. While the geometry shows promise for assessing the shear performance of composite materials, further experimental characterization must be completed to fully vet this novel approach as a standardized shear test method.
Characterization of Damage Evolution as a Function of Material Architecture: A Computational Model Comparison
The ever-increasing use of composite materials in aerospace structure is well documented due to their high strength-to-weight ratios. However, an important design limitation associated with utilizing composite materials is the uncertainty of how damage will evolve in components under in-service conditions. Compound this problem with the various material forms (unidirectional vs. fabric) and the difficulty of predicting damage evolution increases exponentially. This necessitates the development of a predictive framework that is capable of simulating composite damage evolution in a wide variety of load conditions and form factors. This need can potentially be met by BSAM, a progressive damage analysis (PDA) framework developed by AFRL/UDRI/UTA with an extensive history in modeling damage evolution in composite materials. In this work the damage evolution and mechanical properties of Hexcel’s HexPly® M-21 carbon/epoxy were investigated. A variety of ASTM standard tests were conducted to produce mechanical properties for the computational models. In addition, notched over-height compact tension (OCT) and compact compression (CC) specimens were fabricated and tested to evaluate damage evolution for comparison with the computational predictions. Various ply sequences were explored using both unidirectional and woven materials. For the sake of comparison, ply sequences were chosen such that a woven layer in the woven composite corresponded to two orthogonal unidirectional plies in the unidirectional composite. Surface damage evolution was monitored with digital image correlation (DIC), and interior damage
Elucidating the Impact of Microbial Interactions on Fiber-Reinforced Polymers Composites
Fiber-reinforced polymer composites (FRPC), especially vinyl ester-based polymer composites, have become an attractive option for lightweight structural applications due to their excellent moisture-resistance properties. Degradation of FRPCs due to environmental factors such as moisture and temperature are well studied and recognized in the literature. Di-ionized (DI) water and synthetic seawater have been primarily considered to evaluate polymer composite degradation. Little to no attention has been paid to microbes present in natural environments, and their interactions with polymer composites that can potentially aggravate degradation. A few investigations have explored specific fungus or microbial environment to understand their effects on polymer composite degradation. However, no previous study has explored the influence of naturally occurring diverse microbial communities in the environment, such as soil or marine waters, on the performance of polymer composites. Therefore, in this study, we elucidate microbial induced damage on polymer composite materials from naturally occurring diverse microbial communities in natural soil, and further establish their impact on the physical and structural integrity of these composites. Due to the diverse nature of the microbial community, the observations and conclusions from this study can be extended to other natural environments. Carbon fiber reinforced vinyl ester composites were subjected to DI water, autoclaved and ad received soil water for up to 20 weeks. We periodically measured their nanomechanical (nanoindentation) and thermal properties (Thermogravimetric analysis), which showed a reduction these properties in the presence of microbes. Reduction in thermal properties indicated polymer chain scission in composite samples, which was related to hydrogen bond formation through Fourier-transform infrared spectroscopy measurements. Reduction in the amount of dissolved organic carbon content in soil water with time indicated the consumption of carbon by microbes. Additionally, analyzing the biofilms grown on polymer composite samples using metagenomics in collaboration with microbiologists, we confirmed microbial growth. In summary, an in-depth study was conducted to elucidate degradation mechanisms of polymer composites due to microbial activity.
Damage Detection in Polymer Matrix Composites by Analysis of Polymer-Water Interactions Using Near-Infrared Spectroscopy
Detection of early stage damage in polymer matrix composites (PMCs) is a challenge for current non-destructive examination (NDE) methods. New techniques based on sub-micron scale effects may enable identification and characterization of early, non-visible damage. Moisture, ubiquitous in most service environments and readily absorbed by PMCs, may present an opportunity for identifying and characterizing such damage. Water molecules absorbed naturally within PMCs exist either as ‘bound water’ (interacting with the polymer matrix via secondary bonding mechanisms), or as free water (having negligible external bonding interactions). Early-stage physical damage within a PMC takes the form of nano to micron scale free volume within the polymer matrix. This new free volume represents an opportunity for water to occupy and form clusters within the host polymer without directly interacting with the polymer matrix chemically or physically. Therefore, relative to a pristine region, a damaged region would contain a much higher concentration of free water. This altered spatial distribution of free water creates an opportunity for damage identification and characterization. This study explores this possibility by investigating the moisture state in undamaged and 3-joule impactdamaged 16-ply epoxy/glass fiber composite laminates. The specimens are exposed to moisture via immersion in deionized water to simulate long-term moisture exposure in a humid environment. The state of absorbed water is characterized quantitatively via near-infrared spectroscopy. Results show a significantly higher ratio of free-to-bound water in the 3-Joule damaged specimen when compared to the undamaged specimen at the same moisture content. This indicates the existence of free water within voids associated with matrix micro-cracks and delaminations created due to impact. Results from this study suggests that analysis of the nature of polymer-water interaction could be leveraged in developing a technique for damage detection even at very low damage levels in PMCs.
Effect of Moisture Absorption on the Fracture Toughness and Morphology of Halloysite Nanoclay-Epoxy Nanocomposites
Epoxy resins are being used in a wide variety of experiments due to the versatility of the material and the various modifications using nano-scale fillers in the epoxy. These resins have a highly cross-linked molecular structure that lead to them having characteristics such as high strength, high creep resistance, low shrinkage, and good resistance to chemicals. The hydrophilic nature of epoxy resins tends to cause them to absorb moisture. Several studies have shown that moisture absorption in epoxy polymers can lead to both reversible and irreversible changes and resulted in degradation of the mechanical properties. However, in the open literature, there are still critical issues related to the effect of moisture absorption on mechanical and chemical composition of nanomodified epoxy. In this work, the effect of moisture absorption from hygrothermal aging on the fracture toughness and morphology of halloysite nanoclay modified epoxy was experimentally investigated. In order to investigate the physical-chemical evolutions and their influences on the fracture toughness and morphology properties, a comparative study was carried out for both unaged and aged specimens. The specimens were weighed prior to and after undergoing hygrothermal aging. SEM was also used to investigate the effect of moisture absorption on the morphology of the specimens. Preliminary results showed that aging time and temperature have a significant influence on the mechanical and morphology of the nanocomposite materials.
Effect of Fiber Packing on Moisture Diffusivity and Tortuosity in Fiber Reinforced Composites
Carbon Fiber Reinforced Polymer (CFRP) composites are widely used in marine environment due to their high specific strength, increased durability and corrosion resistance property. But these materials when subjected to seawater absorb moisture that results in the reduction of material strength due to matrix plasticization and reduced interfacial strength due to de-bonding at the fiber-matrix interface. To ensure long-term durability and damage tolerance, it is important to understand the impact of moisture diffusion and moisture-induced damage in composites. To that end, the goal of this paper is to elucidate the importance of fiber packing and orientation on moisture diffusion such that CFRP composites can be optimally design for marine applications. In this paper, we developed computational models within finite element method to study the long-term response of CFRP composites subjected to moisture conditions. We developed twodimensional micromechanical diffusion models with varying fiber volume fraction and different fiber distribution like square, hexagonal and random array to study the influence of fiber packing on the effective diffusivity and moisture saturation. Specifically, we elucidated the influence of tortuous diffusion pathways that manifest due to these micromechanical geometric properties of CFRP.
Elevated-Temperature Thermal Conductivity for PEEK-Matrix Composite with Carbon Fibers, Graphite Flakes and PTFE
An investigation on elevated-temperature thermal conductivity of PEEK-matrix composite containing PTFE particles, short carbon fibers and graphite flakes has been conducted. In the experimental phase of the study X-ray diffraction (XDR) is used to determine the morphology of neat PTFE and PEEK, PTFE/PEEK blend and the PTFE and PEEK phases in the CF/Gr/PTFE/PEEK composite. Optical and scanning electron microscopy work is performed to analyze the composite microstructure. The geometry, size, dispersion, and interfacial bonding of the fillers are assessed. The effective thermal conductivities of the composite and the neat polymers are determined by a modulated differential scanning calorimetry (MDSC) method with temperature ranging from -10 to 215°C. The results show that thermal conductivity of the composite with an increasing amount of graphite is affected by temperature and polymer transitions. In the theoretical phase of the study thermodynamic considerations are addressed with theoretical modeling. Different micromechanics models are used to determine the effective thermal conductivity of the composite. The thermal percolation results are compared well with experimental data. The results also indicate that thermal conductivity of the CF/Gr/PTFE/PEEK composite is affected by temperature and the volume fraction of graphite with a threshold 10% (by volume) where percolation occurs due to increasingly enhancing efficiency of thermal transport.
Effects of Graphene Nanoplatelets on the Lightning Strike Damage Response of Carbon Fiber Epoxy Composite Laminates
With the increasing use of carbon fiber epoxy composite materials in the aerospace and wind energy industries, protecting these composite structures from lightning strike has become a vital and challenging task. Conventional lightning protection of carbon fiber composites requires the use of a surface layer of expanded metal mesh film. Such solution has shown disadvantages including (i) additional weight and cost and (ii) debonding between the mesh film and the composite laminate due to environmental aging. An alternative solution is adding lightweight conductive additives to epoxy resin instead of using heavy metal mesh. In this paper, we investigated effects of premixing highly conductive graphene nanoplatelets with epoxy resin on the overall electrical resistance and lightning damage response of composite laminates. Four carbon fiber composite laminate panels were fabricated using hand-layup followed by hot pressing, including two baseline panels without graphene nanoplatelets, two other panels with 0.5 and 1.0 wt. % graphene nanoplatelets, respectively. The quasiisotropic composite laminates were subjected to simulated lightning strikes with 100 kA impulse current. Both visual observation and ultrasonic inspection were carried out to identify the lightning strike damage and characterize the role of graphene nanoplatelets in mitigating lightning strike damage. Our results indicate that adding nanographene platelets significantly decreases the surface electrical resistance of the composite laminate and reduces the severity of the lightning strike damage.
Novel Testing Technique to Evaluate the Durability of Textile Composites at Extreme Environments
At the 35th Space Symposium on April 9, 2019, NASA proposed a plan to return astronauts to the moon’s surface by 2024 [1]. At the moon, the launch vehicles will be exposed to extreme environments including but not limited to lunar dust, extreme temperatures, launch loads, stresses, rapids accelerations, and thermal gradients. Composite materials have become an attractive option to use in aerospace structures. These conditions can degrade the mechanical properties of composite materials, compromising the safety of the crew and the space shuttle. Therefore, it is critical to establish a detailed understanding of the mechanical behavior and damage mechanisms when exposed to these conditions. This study aims to investigate the tensional and flexural behavior of woven carbon/epoxy and woven Kevlar®/epoxy in extreme environments. To accurately simulate the material/mechanics conditions that an aerospace structure will experience, only one surface of each composite is exposed to - 60°C, while the other surface is at 25 °C. An environmental chamber was designed and manufactured to keep these temperatures. The samples were conditioned for 5 hours, after 40 min, the temperature on each of the surfaces reached equilibrium, and for the rest of the time did not have a significant variation. The load applied during the flexural tests was on the surface exposed to -60 °C. For comparison, the flexural and tension tests were also performed on samples exposed completely to 25 °C and -60 °C.
Low Cost Carbon Fiber as Potential Lightning Strike Protection for Wind Turbine Blades
Until recently, glass fiber composites (GFRP) were the preferred choice to prepare wind turbine blades due to their low cost compared to their counterpart carbon fiber composites (CFRP). However, to harvest the maximum wind energy, ever larger wind turbine blades are being manufactured. To support such a large structure carbon fiber composites CFRP have become the integral part of load bearing structures in the blade. In this work, we are proposing to utilize the low cost carbon fiber (LCCF), manufactured at Carbon Fiber Technology Facility (CFTF) of Oak Ridge National Laboratory (ORNL) as not only the cost effective alternative to the currently used carbon fiber (CF) but also as the lightning strike protection of the wind turbine blades. A textile-grade precursor was used to prepare LCCF. Wind turbines often get hit by lightning strikes due to their operating locations. LCCF can provide structural integrity to these gigantic structures and mitigate the effect of lightning strike on them by effectively dissipating the current. Two composite panels made of LCCF were tested against artificial lightning strikes of 100 kA and 200 kA (component A of lightning
Micromechanics Modeling of Polymer Matrix Composites Undergoing Pyrolysis
In this work, a micromechanics-based framework for determining hightemperature thermophysical and mechanical properties of CFRPs undergoing thermal decomposition has been further developed. The goal is to develop a framework that enables to investigate the effects of the mass loss and material phase changes (i.e. formation of the secondary char and gas phases), heating rate, decomposition reaction, and pyrolysis pore pressure on the overall material properties. The present work is mainly focused on the investigation of the effects of the heating rate and decomposition reaction. Temperature- and heating rate-dependent volume fractions of the decomposing matrix constituent phases (polymer, char, and pores) are determined from the firstorder Arrhenius kinetics. Representative volume elements (RVEs) with microstructures corresponding to specific temperature and heating rates are created and two-step numerical homogenization of RVEs has been performed to determine overall material properties of the AS4/3501-6 composite in a temperature range up to 900 K. It was determined that overall material properties exhibit strong dependence not only on the temperature but also on the heating rate. An increase in the heating rate shifts initiation of thermal decomposition to the higher temperature. As a result, composite properties are retained at the higher temperatures.
Considering the Randomness of Mechanical Properties in Simulations of Discontinuous Fiber Reinforced Composites
Discontinuous Fiber Reinforced Polymer (DFRP) composites often display large variations in mechanical properties and large difference between tensile and flexural strengths. This work investigates a glass fiber Sheet Molding Compound (SMC) composite. The experimental results show that the flexure strength is 86% higher than the tensile strength. Using the mean mechanical properties measured in tensile, compression and shear tests would result in underprediction of the flexural behavior. The randomness of the tensile strengths of DFRP composites is discussed. A Weibull distribution model comprising of both the tensile and flexural strengths is proposed. The use of this model in FEA is demonstrated.
Experimental and Numerical Study of Damages Induced by Low-Velocity Impact on 4th Generation Composite Laminates
The use of composite fiber reinforced polymer (CFRP) structures in the aircraft industry needs to demonstrate that they satisfy the same safety requirements as for metal structures. Among these requirements, impact damage tolerance is a major sizing criterion for those structures. Impact induce complex out-of-plane loadings that results on large damage extensions. Experimental campaigns are realized at the industrial level to design composite structures but, it induces long and costly development for innovative solutions. Finite Element Model (FEM) is a good tool for sizing efficiently composite components even those exposed to impact. Unfortunately, numerical models developed by academics are too complex to be used in the industry. Besides, due to the highly non-linear problem, explicit solvers are used to avoid convergence issues, which require an expertise in the results verification. In this way, we present a damage model developed at the ONERA [1,2] to predict the progressive rupture of last generation composite laminates subjected to low-velocity impact (LVI)/low-energy impact (LEI). This model has been developed with a fair level of complexity in order to be used in aeronautical industries. This material has thick resin layers at each interface between each plies, which increase the toughness of the laminate. In order to understand damage mechanisms of this specific material when subjected to LVI/LEI, an investigation of impact damages has been carried out in this study. Since LVI/LEI do not enter in the scope of fast transient dynamics, we use an implicit integration scheme and the first results using the damage model are compared with the experimental data given by experimental testing.
Increasing the Translaminar Fracture Toughness of Carbon Fibre Composites by Hybridising with High-Performance Polymer Fibres
The translaminar fracture toughness governs the damage tolerance and notch sensitivity of a composite material. A promising strategy to improve the translaminar fracture toughness is to hybridise brittle carbon fibres with tough fibres, such as highperformance polymer fibres. These fibres introduce an additional energy dissipating mechanism by fibrillating upon failure. However, the translaminar fracture toughness performance of the composites with these fibres has not yet been extensively reported in the literature. Compact tension tests were performed to analyse the translaminar fracture toughness of aramid and polyarylate (PAR) fibre composites. Carbon fibre composites were used as the reference. The laminates were produced by manufacturing prepreg and subsequently curing them in an autoclave. An appropriate sample design was set-up to perform the compact tension tests. The propagation translaminar fracture toughness of the PAR fibre composites was 492 kJ/m², significantly higher than the 39 kJ/m² of the carbon fibre reference. In contrast, the aramid fibre composite did not show a significantly higher toughness than the carbon fibre reference. Different design strategies have been examined. Having a strip of PAR fibres proved successful to locally slow down crack growth by increasing the translaminar fracture toughness. The results presented in this work allow to further optimise the design of composites for damage tolerance. These improvements can increase the safety of composite parts for society while also decreasing the weight of the part.
Characterization and Modeling of Blast Resistance Behavior of Carbon Composites
The vulnerability of aerospace structures subjected to near field blast loading plays a critical role for aviation safety and security evaluation. The primary objective of a recent collaboration study conducted by the Transportation Security Laboratory (TSL) of the Department of Homeland Security and the Combat Capabilities Development Command (CCDC)-Army Research Laboratory (ARL) was to perform close-in blast experimental characterization and computational modeling of dynamic deformation and damage progression of aerospace materials. The results of this experimental/numerical study for lightweight materials, including aluminum and carbon composites are presented in this paper. A recent paper [1] reported that a series of close-in shock-hole blast tests on carbon composite panels were tested and correlated with simulated results using the LS-DYNA®- Arbitrary Lagrangian-Eulerian (ALE) method. The computational constitutive model integrated within LS-DYNA® was validated to characterize the progressive failure behavior in carbon laminates subjected to close-in blast loading conditions with reasonable accuracy. However, the ALE approach is computationally expensive and limited to be used for complicated aerospace composite structures consisting of composite panels and stiffeners. Thus, in this study, a simplified approach for predicting blast loading using ConWep code has been selected and validated for the vulnerability evaluation of aerospace structures using composite materials.
Assessment of Material State for Predicting the Durability of Composites
The long term behavior of composites have been extensively studied for the last four decades. Given the heterogeneity of these materials, the damage accumulation mechanisms lead to superior fatigue performance of composites compared to metals. However, due to the ‘sudden death’ behavior controlled by defect coupling, the precursor to fracture plane development of these materials, the challenge remains on how to assess the real-time material state and predict when it becomes critical? In the recent past, broadband dielectric spectroscopy (BbDS) has been used successfully to assess the material state and predict the material state change (triggered by defect coupling) for quasi-static loading. In this work, we perform insitu monitoring of material state change for tension-tension fatigue loading (low cycle fatigue) and attempt to capture the material state change for quasi-isotropic laminates. The insitu response can be used to predict the formation of a ‘critical’ material state- the final frontier to predict the durability of composites and discuss the repeatability of the methodology.
Investigating Transverse Ply Thickness and Cracking Effects on the Tensile Strain to Failure of Thin-Ply, Cross-Ply Carbon Composites Using X-Ray Computed Tomography
The effect of 90°-ply thickness on the fibre direction tensile failure strain of crossply laminates was investigated by designing four different cross-ply composite laminate configurations with 90° blocks of varying numbers of plies adjacent to a central block of 0° plies. The range of 90° block thicknesses evaluated included conventional ply thicknesses to allow for comparisons with conventional ply geometries. The central 0° ply block was subjected to primarily longitudinal tension with a small component of transverse tension due to the layup and thermal residual stresses. Advanced instrumentation, including acoustic emission and X-ray computed tomography, was used to detect and visualize damage accumulation in the specimens. It was found that decreasing the thickness of the 90° ply blocks reduced transverse matrix microcracking up to the point where no full-width transverse cracks developed prior to ultimate failure. A small degradation of the fibre direction failure strain in the central 0° plies was found as the 90°-layer thickness increased in the laminates, which corresponded with the development of edge/transverse cracks.
Integrated Simulations for the Structural Performance of Composites
Fiber-reinforced composites are widely used for structural components in the aircraft, automotive, marine, and other industries due to their low density, high specific stiffness and strength, excellent durability, and design flexibility. During the fabrication of continuous fiber reinforced composite components, fiber direction changes, residual stresses, and out-of-the-mold deformations are unavoidable. As the 2-D fabric is deformed to a 3-D part geometry the fiber tows move, leading to fiber direction changes that result in relative angle changes between the fibers that make up the fabric. These changes have a significant effect on the behavior of the composite material system. The extent of fiber angle change in a non-crimp fabric system is largely dependent on the differences between the 2-D and 3-D geometries and the particular stitching parameters used to manufacture the fabric. In order to better understand the influence of fabrication induced fiber angle change on the performance of structural composite parts, detailed experiments and simulations were conducted. For the experiments, a reinforcement geometry was selected. Due to the complexity of the chosen geometry for draping, several slits were designed at strategic locations to allow the fabric to take the shape without wrinkling. During molding, two patterns were overlaid so that the slit locations after molding were staggered through the thickness to reduce their effect on the structural performance. Detailed draping analyses were performed taking into account the process steps, and the fiber angle changes were calculated using numerical models that were developed previously. Further, the fiber angles following draping were mapped onto the structural performance models used to simulate the crush tests. The predicted stiffness and strength results from the integrated fabrication and performance simulations were compared with the experimental measurements, and the correlations are presented in this paper.
Multiscale Modelling of the Cure Process in Thermoset Polymers Using ICME
An Integrated Computational Material Engineering (ICME) approach is employed to predict the thermo-mechanical properties of thermoset polymers at different stages of cure. The framework integrates Molecular Dynamics (MD) at nanoscale and Finite Element Methods (FEM) at microscale. MDis used to predict the volume shrinkage and mechanical properties of an epoxy resin as a function of the crosslink density at room temperature using the Interface Forcefield (IFF). The predicted resin properties show good agreement with the literature values, showing that IFF can be reliably used for this purpose. Once characterized, the predicted properties can be used to further predict the effects of cure shrinkage on bulk-level composite residual stresses.
Multi-scale Approach to Predict Cure-Induced Residual Stresses in an Epoxy System
This article details the modeling of the EPON862/DETDA epoxy system using Molecular Dynamics to predict the mechanical properties as a function of crosslinking density. The Reactive Interface Force Field (IFF-R) is implemented in this work. The properties of the epoxy system were predicted at different crosslinking levels. The predicted density, shrinkage, and bulk modulus showed a good agreement with other modeling and experimental data in literature.
Impact Damage Tolerance Enhancement of Woven Composites at Extreme Environments
According to the US Geological Survey, 22% of the world’s undiscovered resources (natural gas, oil and natural gas) are located in the Arctic. As global temperatures rise, larger windows of arctic water routes are becoming viable. The materials used in the ships that will navigate the Arctic must be able to withstand harsh environments that the arctic routes may bring, with temperatures dropping to -60 °C and ice projectiles impacting the hull. Therefore, when designing ship hulls, the materials selected must be able to withstand thermo-mechanical loads. Polymer matrix composites (PMC) have become a potential option to use in ship hulls due to their high strength-to-weight ratio. However, their mechanical behavior in extreme environments is still unknown. This study investigates the impact behavior of PMC reinforced with finely ground Kevlar pulp as an interlaminar reinforcement to increase the impact damage tolerance at extreme environments. The laminates were manufactured via resin transfer molding (RTM) in a compression press. The dimensions of each sample were 150 mm long by 100 mm wide and a thickness of 4 mm. The impact tests were performed in an Instron CEAST 9340 drop weight impact system using a mass of 5 kg and impact energies of 5 J, 7.5 J, and 10 J, for room and arctic temperatures (-60 °C). Reinforced samples (Kevlar) showed a decrease on the degree of damage (ð·ð‘‘ð‘Žð‘šð‘Žð‘”ð‘’) of ≈20%, a decrease on the bending stiffness of ≈17%, an increase on the peak impact force of ≈5% and a decrease on the displacement of ≈12%. Preliminary results showed that laminates reinforced with Kevlar have higher impact damage tolerance than unreinforced samples.
Hail Impact Testing of Stitched Carbon Fiber Epoxy Composites Laminates
The hail impact response of the composite laminates is crucial in providing design guidance on the minimum allowable gauge thickness of the novel Pultruded Rod Stitched Efficient Unitized Structure (PRSEUS) composite panels for next-generation aerospace structures. In this work, we performed simulated hail impact tests for stitched and non-stitched flat carbon fiber epoxy composite laminate panels to study their different damage behaviors. All panels consist of a mid-bay area and four stiffening strips representing the aircraft composite structure stiffened with stringers. The tests were conducted using a gas gun and ice ball projectiles with a diameter of 40 mm. Different hail impact speeds (40 - 77 m/s), and hence different impact energies were imposed through tuning the pressure of the gas gun. After the hail impact tests, both visual observation and ultrasonic C-Scan were conducted to observe the surface damage and internal delamination. It was found that the matrix cracking occurs in every composite panel along the edges of the stiffening strip. In addition, delamination is found for both stitched and non-stitched panels in the mid-bay area and stiffening strips. The delamination area in the mid-bay area for the stitched composite panel is smaller than that for the non-stitched composite panel. The delamination in the stiffening strips could be caused by the stress concentration due to the geometric discontinuity between the skin and the stiffening strips. The observed different damage behaviors provided insights into the delamination arrestment capability of the stitched composites.
In-situ Observation of Dynamic Damage Evolution Inside GFRCs Through High-Speed X-ray PCI
We visualized, in real time, the dynamic damage evolution inside glass fiber reinforced composites (GFRCs) subjected to low-velocity impact by high-speed X-ray phase contrast imaging (PCI) technique. Dynamic single-edge notched bending (SDENB) experiments were performed on a modified Kolsky compression bar. The specimen was mounted on a fixture at the end of the incident bar, impacted onto an indenter pin on a fixture installed ahead of a load cell which was used to replace the transmission bar. During the dynamic loading, synchrotron X-ray penetrated the specimen, detecting the internal damage of the composite material. The X-ray signal was then transferred to visible light through a scintillator and finally recorded by a high-speed camera. Different failure behaviors of unidirectional and cross-ply composites with varied ply thickness were studied. It is revealed that high-speed X-ray PCI technique can visually identify the crack initiation, crack propagation within and among multiple plies and various micro-scale damage mechanisms in high-resolution of 10-micron-meter scale.
Quasi-Static and Dynamic Analysis of Composite Panels: Challenges in Engineering-Level Commercial Software
In engineering applications, where analysis time and efficiency are essential, analytical models and commercially available finite element (FE) material models are the two mainstream methods used to analyze composite crush scenarios. A reliable numerical model addressing the crushing response of composite materials requires a nonlinear progressive damage analysis which includes appropriate failure mechanisms such as matrix cracking, fragmentation, delamination, and fiber breakage. One might be able to address all these interacting failure responses through implementation/validation of a user defined material model or by utilization of a commercially available model, the latter being the most efficient engineering approach. In this paper, the quasi-static and dynamic crushing response of composite specimens, consisting of a C-channel and a corrugated geometry manufactured with carbon fiber/epoxy matrix fabric, are modeled using the progressive composite damage model within ABAQUS. The novel part of this work, which has not been previously addressed in the literature, is the utilization of a detailed parametric study to optimize composite damage model parameters for a given quasi-static crush simulation in ABAQUS. C-channel experiments were used to calibrate the model and then refine/optimize a set of material parameters that best fit the experimental results. These identical parameters were then used to simulate the crush response of the corrugated channel specimens, thereby making it a pure prediction and not a double recalibration. Next, the dynamic response of both panels were predicted, further confirming the predictive capability of the developed numerical model. The effect of parameter variation on the crush response is fully documented for future use in engineering applications utilizing this specific ABAQUS damage model.
Impact Dynamic Behavior of Soft Composites at Low Temperatures
Dynamic testing soft composite, matrix, and other hyper-viscoelastic materials at high velocities replicating impacts is difficult to perform, either due to the lack of equipment such as a split Hopkinson pressure bar setup, or more commonly, the challenges related to sensors and monitoring the dynamic response. For certain soft composites and hydrogel matrices with low moduli on the order of 215,000 Pa, sensing would require a highly sensitive load cell that would also need to survive >1,000 s-1 without failure and/or without digital image correlation allowed as paint or other speckles may not adhere to the specimen surface. Our current research addresses this issue by examining the temperature effects on the material response. By exploring the interaction between temperature and viscoelasticity, we have validated a theoretical framework for performing up to two decades of strain rate testing (100 – 10,000 s-1) without altering the impact velocity (7 m/s), and using only a 14 °C temperature change. The implications of this research should allow us to examine strain rate effects for certain soft polymer composites at loading rates exceeding the capabilities of our instruments without any new equipment or sensors.
Dynamic Fracture of Thermosetting Resin System for Additive Manufacturing of Advanced Composites Using Digital Light Processing
This study examines the dynamic fracture behavior of high-performance thermosetting polymers for additive manufacturing (AM) to enable rapid production of lightweight and functional polymeric and composite materials that meet military requirements. We present a systematic investigation of the role of print orientation on the dynamic fracture response of a bis-GMA based AM polymer as a model material, using single edge notched tension (SENT) geometry. While most fracture criteria in predictive simulations use quasi-static values, fracture is innately dynamic and impulsively loaded cracks can have toughness values that differ by more than 20%. A unique long-bar striker apparatus is used to fire a striker at the opposite end of the notched and pre-cracked specimens in order to create a dominantly dynamic Mode-I (opening) fracture load. Digital Image Correlation (DIC) is used in conjunction with ultra high-speed imaging to capture the evolving displacement, and hence strain fields ahead of the crack tip. The elastodynamic solution accounting for inertial effects and wave interactions is optimized using a least squares fit to extract the evolving critical stress intensity factor (SIF) leading to initiation. Preliminary findings suggest that under impulsive loading, the print orientation does slightly affect the overall dynamic fracture toughness of the material.
Low Velocity Impact Testing of Ice on Steel and Composite Specimen
This study investigates the differences in low-velocity impact response between steel and composite sandwich structure using a customized ice impactor. Different combinations of freezing temperature and duration are used to create the ice impactors. Ice impactors that are frozen at -12ËšC for 48 hours are found to have the least damage from visual observation after low-velocity impact tests on A516 carbon steel. Subsequently, carbon fiber reinforced composite sandwich panels are subjected to the ice impacts. X-ray micro-computed tomography is utilized to reveal any potential inner damage of the composite structure. Different fracture mechanisms of ice are observed for different targets- full damage of ice is more evident for composite specimens while partially damaged ice is dominant in steel specimens. Though higher peak forces value is noticed from the ice-steel interaction, much energy is absorbed in composites during impact testing. Discrepancies in displacement of the ice impactors with test duration are also observed with the variation of the targets. Furthermore, size and dimension of the broken pieces of ice impactors vary according to the targets. This research will further enhance the knowledge in understanding low-velocity ice impact on composite sandwich panel in extremely low-temperature Arctic conditions.
Experimental Evaluation of Axial Crush Behavior of Woven Tubular Laminates with a Moving Support Condition
The objectives of the investigation were to measure the specific energy absorption (SEA) of the plain-weave carbon fiber reinforced polymer (CFRP) tubes, study the effect of tube length on SEA and stable crushing force, analyze the effect of using a crush trigger, and compare force-displacement data using different specimen support conditions. Dynamic crush testing of CFRP tubes was performed using the crash sled in the Impact Dynamics Laboratory at NASA Glenn Research Center. The sled features a movable mass that supports the specimen during crash testing. Laminated [±45]4 tubes were fabricated and tested using tube lengths of 102 mm or 152 mm each with a diameter of 76 mm. Sled impact velocities ranged from 6.15–6.48 m/s. In most tests, a crush trigger was applied to the front face of the tubes to initiate a crushing failure mode. The mean SEA of the tubes tested with the crash sled was 48.6 J/g. There were virtually indistinguishable changes in failure modes for the 102-mm and 152-mm tubes. Plots of acceleration and cumulative absorbed energy for both tube lengths were nearly identical, which suggests that the effect of tube length on crush behavior was negligible in the range of tested impact velocities. One of the 102-mm tubes was tested without a crush trigger to provide a direct comparison with a drop tower specimen that lacked a crush trigger. A higher peak force and lower SEA was measured for this specimen compared to the specimens that had a crush trigger. Both the peak force and stable crushing force were comparable for the drop tower and crash sled specimens lacking a crush trigger. However, the maximum crush displacement of the crash sled specimen was lower because a portion of the energy was absorbed by the motion of the support mass.
Understanding Post-Impact Compressive/ Bending Performance of Composites Using Linear Elastic Fracture Mechanics Model
In this study, we employ the use of a linear elastic fracture mechanics (LEFM) model to understand and predict the post-impact performance of composites. Compressive-after-impact (CAI) and bending-after-impact (BAI) strength of carbon fiber reinforced polymer (CFRP) composites are important design considerations in the aerospace industry. Although CFRP composites are light-weight and highstrength, their susceptibility to impact results in barely visible impact damage (BVID), consisting of complex damage modes (matric cracking, delamination, fiber breakage, etc.) often hidden beneath the surface of the composite material. BVID subsequently leads to drastic reduction in CAI and BAI strength. We utilize a LEFM model to understand how impact damage affects post-impact performance. In particular, we study how impact damage depth (d), damage width (l) and impact-induced delamination (a) affects brittle fracture of composites during compressive and flexural loading. Strain energy release rate (SERR) is calculated to predict crack growth based on fracture mechanics. Results show that increasing damage depth (d) results in higher SERR. Moreover, decreasing undamaged depth, h, also causes higher SERR, thereby resulting in CAI brittle fracture. Interestingly, when the total depth is kept constant, an exponential increase in SERR is evident when damage depth reaches around 60% of total depth. Results in the analytical models are compared to experimental results of CAI and BAI tests of CFRP composite foam-core sandwich structures, with the aim to study the influence of low temperature Arctic conditions. The motivation of this study is fueled by the recent global trend to explore the Arctic region due to greater accessibility to the area, as new sea routes are opened as a result of Arctic ice melting. Our study demonstrates a significant influence of low temperature on impact behavior and post-impact performance.
Unsupervised Learning Methods for Identification of Defects in Heterogeneous Materials
The complexity of composite materials due to the nature of their numerous laminated layers, stacking sequences, type of fibers, resin, and other external factors has challenged the world of structural health monitoring (SHM) and non-destructive inspection (NDI). Post-processing of these SHM and NDI methods has been mostly a manual time-consuming process, with human inspection causing errors associated with the bias decision made by NDI inspectors. Recent advances also call for Cure On The Fly (COTF), by integrating NDI with advanced technologies and analysis techniques such as Artificial Intelligence (AI) and Machine Learning (ML) for on-line real-time predictions of a defect, damage, strength, and life. Broadband Dielectric Spectroscopy (BbDS) is based on an interaction of the electromagnetic waves with the matter and is identified as a robust tool to extract the material level information based on the morphology changes in them. It assesses the materials state by identifying the micro-defects generated and the orientation of those defects. Previous research has proved that this method can be employed to assess the different material state parameters caused by the different modes of damage, specific defects present in them, and hence predict their strength based on these assessments. In this research work, we integrate the method of using BbDS to obtain the material properties i.e. dielectric spectra of the composites with foreign object defects induced in them and using the state of art unsupervised learning methods of ML to identify them in real-time after manufacturing the materials. This will also serve as the initial step of a framework to predict the other modes of damage based on its materials state. We also propose hybrid methods of using unsupervised learning followed by supervised learning to improve accuracy in predictions and to remove outliers.
Deep Reinforcement Learning for Composite Material Optimization
Structural optimization has long been a complex and time-consuming task as it requires many designs to be verified through finite element method (FEM) prior to achieving an optimal solution. This often necessitates a surrogate method that could extract key features in a structure and determine suitable designs more efficiently. In this paper, we introduced deep reinforcement learning (DRL) by combining deep Convolutional Neural Network with Reinforcement Learning algorithm. To validate the optimization accuracy, the deep Convolutional Neural Network is trained based on four different types of geometric models that are commonly used in the analysis of composite materials: plate with circular cutout model, square packed fiber reinforced, hexagonal packed fiber reinforced and hollow particle reinforced composite models. Our result will show the DRL framework could accurately and efficiently propose the optimal design for different composite models
Learning Composite Constitutive Laws Via Coupling Abaqus and Deep Neural Network
The commercial finite element (FE) code Abaqus is coupled with the deep neural network (DNN) model to form a novel mechanics system, namely Abaqus-DNN mechanics system. The proposed system enables data communication between Abaqus and DNN model, which leverages the versatile FE analysis ability of Abaqus and the powerful machine learning using DNN. Abaqus-DNN enables DNN to learn the constitutive laws in a form-free way and the learned result automatically satisfies the equilibrium and kinematics equations, which avoids inaccuracies associated with the presumed functions in the constitutive laws and guaranteed the learned constitutive law following physical laws. The Abaqus-DNN mechanics system was implemented to learn the full set of engineering constants of the constituents of a fiber-reinforced composite. Furthermore, the proposed system was applied to learn the progressive damage constitutive law of a fiber-reinforced composite laminate. The backward propagation equations of the neural network were modified to track the gradient of the loss function from Abaqus to DNN. The results show that Abaqus-DNN can accurately learn constitutive laws based on partially experimental measurements, which provides a generalized approach for learning unknown physics inside a mechanics system by coupling neural network with commercial finite element codes.
A Framework for Bayesian Calibration of Fiber-Reinforced Composite Material Properties
Fiber-reinforced composite materials continue to rapidly improve in terms of structural performance. These materials offer the promise of significant weight reduction in many products—particularly aerospace structures, where weight minimization is critical to both cost and performance. However, the introduction of new materials is a slow and expensive process that requires dependable material property data early in the design cycle, and it takes a rigorous, comprehensive testing program to fully characterize the properties first at the ply level, then at the laminate level, and finally at the product or component level. The ability to reliably predict laminate material properties using limited ply-level material property data early on— and an understanding of the accuracy of those predictions—can dramatically shorten the design cycle. We propose a data-driven approach to obtain material properties for newly developed composite materials. The proposed approach uses limited ply-level property test data to generate finite element model (FEM) parameters that best fit the statistical distributions from test data. The material model correlation process uses neural networks as surrogates for high-fidelity FEMs. The surrogate model is then used to perform Bayesian calibration to directly find the parameters that produce a best fit for a given set of test data. At the end of the process, statistical distributions of all the parameters of interest are provided. This process provides a tractable and systematic approach for the selection of FEM material parameters for a novel material with limited test data. In recent work, the approach has been validated using test data derived from a composite material typically used in space applications. This technology could potentially enable accelerated development of new materials, and it could be extended to other kinds of materials beyond composites—especially those materials that exhibit high levels of statistical variability.
Parametrization of Reactive Potential using Genetic Algorithm and Machine Learning Techniques
Multi-scale modeling of composites spans many length scales ranging from atomistic to the continuum level. At the lower length scales, classical molecular dynamics (MD) provides insight into the failure mechanisms in the fiber, matrix, and the interphase region that forms during processing. MD requires selection of an accurate potential energy model to describe the system. Proper usage of the potential energy model requires parameter optimization of the model’s numerous parameters with respect to experimental geometric and energetic constraints. Parametrization of a potential model can be problematic due to its time-consuming and labor-intensive nature. The goal of this work is to develop a machine learning inspired evolutionary parametrization technique to decrease the time cost and diminish the human intervention required in the parametrization process. The evolutionary genetic algorithm is employed to optimize the parameters of the ReaxFF interatomic potential. Several augmentations are implemented using machine learning techniques. The utilization of an artificial neural network as a surrogate for the ReaxFF potential is considered. Changes to the genetic algorithm are incrementally benchmarked for accuracy and time cost with respect to a simple zinc-oxide model. Utilizing the artificial neural network significantly boosted performance, as measured by the final total error and the rate of decrease of total error with respect to time. The double- Pareto probability density-based crossover operator and a multiple standard deviation based Gaussian mutation scheme outperform their counterparts. The computational time cost to achieve the same level of accuracy relative to manual training is decreased from months to days.
Optimal Design of Nanocomposite Material and Fabrication Parameters Using Artificial Neural Network
Artificial neural network (ANN) is a mathematical model that maps an ndimensional real input to an m-dimensional output. The basic computing unit, called a node, calculates the weighted sum of the inputs and compares it with a threshold (or bias) and sends a signal to the output after passing it through an activation function. Activation function is the mathematical function that defines the way an incoming signal at a node is treated before being outputted; commonly used functions are linear, tan sigmoid and log sigmoid. The set of nodes that receives the input from the data set and those, which deliver the final predicted outputs are called the input and output layers, respectively. Other node sets, the hidden layers, have no direct connection to the outside but are connected to the input, output and other hidden layers. An ANN model is trained, that is, the weights connecting the various nodes adjusted through an iterative process. We have use an ANN-based model to predict the flexural strength of composites. Experimental data from 96 experiments were used to train and validate the ANN. Four parameters corresponding to different manufacturing quantities have been used as the input parameter, and the two parameters representing failure load and displacement has been chosen at the output of the ANN model. The Backpropagation was used to train the ANN model, while the Levenberg-Marquardt was employed for minimizing the errors. The activation functions used are linear for the nodes in the input layer and tan sigmoid for the nodes in the hidden and output layer. We also found that the amount of data which is needed by the ANN model to obtain similar accuracy results to that of the traditional regression approach is about 40-50% less. This means we should be able to accelerate the process of composite design by almost 2 times. Also it was found that the ANN model predicted similar results, but with higher correlation coefficients and R2 values compared to the values available for reference. Additionally, we found that the model needed only 50% of the original data to produce results with similar correlation coefficients and R2 values as the values available for reference.
In-Situ Characterization of Polymer Matrix Composites and Progressive Damage Analysis of Virtually Reconstructed Microstructures
Process-induced variabilities in fiber-reinforced polymer composite (FRPC) microstructures result in large scatters in the experimentally measured composite properties which lead to conservative designs and unnecessary manufacturing cost. Processing conditions such as cure temperature and time combined with low thermal conductivity impose spatially varying thermal histories that influence cure evolution and property development in composite during manufacturing. Additionally, irregular and off-axial fiber architectures introduce thermo-mechanical constraints which lead to residual stress generation, damage accumulation during curing, and in-situ matrix properties. Complete understanding of the process-induced variation in the fiber and matrix at the microscale is necessary to identify sources of scatter in bulk composite mechanical properties and eventually optimize the manufacturing process. The objective of the proposed study is to establish the processing-microstructure-property correlation to improve predictions and reduce processing times of FRPCs using experimental techniques and computational modeling. Statistical descriptors are developed to study process-induced variation in the fiber microstructures. Micro-CT scans of aerospace- and automotive-grade laminates are analyzed. The effect of processing conditions on the two microstructures is investigated; qualitative and quantitative comparisons of the statistical metrics are made. Micro-Raman spectroscopy is successfully employed to generate a 3D map of the degree of cure variation in a partially cured epoxy sample. This technique will be further explored to quantify the process-induced degree of cure variation in a composite laminate. Finally, a finite element (FE) based process modeling computational framework is developed to virtually study the process-induced variations in the composite microstructure. The process-induced variability in fiber and matrix will be correlated to residual stress generation, damage accumulation during cure, and bulk composite properties through process modeling simulations of virtually reconstructed microstructures.
Analysis of Composites Reinforced with Functionally Graded Inclusions
In this paper, the thermal stress field is derived for an elastic medium that contains a spherical inclusion made of a functionally graded material (FGM) where the material properties vary continuously. A uniform heat source in the inclusion generates a temperature distribution which causes thermal stresses due to the thermal expansion. The temperature field is derived analytically. It is shown that only a single ordinary differential equation is needed to express the thermal stress which is solved numerically using the least square method. Once the stress field for a single inclusion problem is obtained, it is possible to extend the result to a composite with distributed inclusions using a variety of methods available in micromechanics.
Statistical Descriptors of Composite Fiber Aggregation
This study introduces a method of characterizing fiber aggregation and resin rich regions in composite microstructures. Microscale models of representative elements (RVE) need to be indicative of the extend of clustering (i.e. close fiber-to-fiber interaction) and resin rich “poolsâ€ which may impact the overall strength and performance of a composite structure. This algorithm was used to evaluate different unidirectional 2-D microstructure scans, which will be compared to their manufacturing method or any special treatment processes. These cluster and pool scan statistics can be used as criteria to judge statistical equivalency of artificially constructed microstructures.
Interfacial Shear Strength (IFSS) and Absorbed Energy Versus Temperature in Carbon Fiber-Thermoplastic Composites via Single Fiber Pullout Testing
Single-fiber micromechanical experiments are often conducted to obtain interface properties between the matrix and fiber in fiber reinforced composites. This study investigates the apparent interfacial shear strength in AS4/polypropylene composites using the single fiber pullout test at sub-ambient, ambient, and elevated temperatures. Fiber failure surfaces are examined (using scanning electron microscopy) to identify the failure mechanisms present and embedded length of the fiber. The results indicate a higher IFSS at low temperatures than at elevated temperatures. Specific energies associated with debonding frictional sliding are also shown to be inversely temperature dependent. The failure mode transitions from cohesive failure in the matrix at room temperature and elevated temperatures to interfacial failure at the lowest temperature. This experimental setup and results will help to understand the temperature dependent failure mechanisms in polymer composite systems across a range of temperatures.
Tailoring the S-2 Glass/Epoxy Interface Properties Through Chemical Vapor Deposition of Silane Adhesion Promotors
This work investigates the use of an ambient temperature chemical vapor deposition (CVD) process to deposit thin layers of (3-glycidoxypropyl) trimethoxysilane (GPS) onto the surface of unsized S-2 Glass fiber. The duration of silane vapor deposition was changed to vary the coverage and thickness of the silane layer to determine the effect of the interface thickness on the interfacial properties. The interfacial shear strengths between these coated fibers and DER353 epoxy cured with Amicure PACM were measured using a single fiber pullout test. These test results suggest that the IFSS can be increased by 50% compared to the unsized baseline and controlled by changing the CVD exposure time. This will allow for the tailoring of the interfacial properties and give materials engineers an opportunity to further optimize the composite material for their applications.
An Extended Annular Coated Inclusion Model with Imperfect Interfaces
Interactions between the nanoinclusion and polymer matrix in nanoparticlereinforced polymer nanocomposites result in an interphase region with different properties than the bulk polymer. This polymer interphase and complicates micromechanical predictions of effective properties and may have a large influence on the overall nanocomposite properties. Recently the Annular Coated Inclusion model has been formulated in terms of appropriate single-inclusion auxiliary problems following the procedure of Benveniste, such that the components of the dilute strain concentration tensor can be determined analytically for coated spherical and coated cylindrical geometries. These solutions then available to be directly implemented within standard micromechanical models. This framework can be extended by implementing a spring-layer model incorporated within the Annular Coated Inclusion framework to evaluate the impact of an imperfect interface on these micromechanics model predictions. To verify the accuracy of the approach for the spherical inclusion case, the interfacial stresses predicted by the proposed model are shown to be identical to classical analytical results, while the effective composite elastic modulus predictions closely agree with results generated by a thin-soft interphase model. Lastly, one can show that the imperfect interface model can be extended for non-dilute particle concentrations via iterative solution of the appropriate auxiliary problems. In this manner, one can determine the stress fields within the composite as a function of the spring-layer interface model parameters in a way that accounts for the impact of particle-particle interactions on the composite response. The stress distributions for cases of both shear and uniaxial loading agree closely with a corresponding Finite Element analysis which highlights the utility of the proposed approach.
Effect of Local Fiber Volume Fraction Variation on Stress-States in a Twill Orthogonally Woven Textile Composite
In this paper, we developed a model for the mesoscale elastic properties for tows as a function of the fiber volume fraction and studied the effect of accounting for the variation of the local fiber ð‘‰ð‘“ in the tows on the predicted stress concentrations in each type of constituent. We used ensembles of microscale fiber/matrix analyses to develop a model of the mesoscale properties as a function of the local fiber volume fraction. We used VTMS to create a twill orthogonally woven 3D textile composite and performed two mesoscale elastic analyses: one with and one without the variation of fiber volume fraction included. In both simulations, the textile was subjected to tension along the warp direction. The results showed up to a 20% difference between the two simulations for the most severe stresses in the binders. In warps and wefts, the stress concentrations differed up to 5%, and in the matrix, the stress concentrations differed between 2% and 6%. The results showed that accounting for the variation of the local fiber volume fraction in the tows could significantly affect the predicted initiation of damage, but reliable quantitative predictions will require additional experimental data that quantify the relationship between fiber fraction and allowables.
Brazilian Disc Test for Polymer Matrix Composites
The Brazilian disc experiment has been used to indirectly measure the tensile properties of heterogeneous materials. Due to the fibrous nature of polymer matrix composites, the random distribution of fibers, shape, and size could influence fracture initiation and propagation. The Brazilian disc could be utilized to understand how the microstructure influences the failure behavior of interfacial debonding and matrix crack formation. Furthermore, damage propagation in many microstructural experiments can be extremely catastrophic and the need for a slow growth crack propagation methodology would be extremely beneficial to the micromechanics community. Two cylinders of 1.6 mm and 2.75 mm diameter Brazilian discs were designed to undergo testing to extract indirect tensile data. Additional experiments were conducted to capture displacements and strain via digital image correlation using stereomicroscopy. These two different size cylinders were obtained from IM7/977-3 unidirectional laminates [0]24 via core drilling through the side of the panel. The purpose of this research is to determine the efficacy of the Brazilian disc in relation to understanding how the microstructure plays a role in the failure of composite materials. Discrete damage modeling approaches also aided in understanding how failure occurs given the stress state of the material.
Thermo-Mechanical Geometry Generation for a Mesoscopic Simulation of a Triaxial Braid Carbon Fiber Reinforced Polymer
In this study, an artificial thermal-mechanical method for generating an accurate and penetration free geometry of a triaxial braid fabric is developed. The quasiisotropic properties of triaxial braided fabric allow for more balanced and homogenized behavior compared with bi-axial woven fabrics. Moreover, the unidirectional and off-axial reinforcements in one layer increase mechanical stiffness properties and stability in the laminate. Simulation methods for predicting braided properties are vital for designing the use of triaxial braided fabric in industry. Based on the dimensions of a measured 0o/+60o/-60o braid architecture, an artificially thinned yarn geometry is defined using python scripting in the TexGen open source user interface for an initial penetration free model. Orthotropic material properties are assigned for the coefficient of thermal expansion and conductivity, and a finite element (FE) simulation is performed in Abaqus to artificially expand the yarn geometry using thermal fields. A post-expansion mechanical simulation compresses the braid to the desired ply thickness, with cross-section rendering confirming good compliance of the generated geometry to measured values. The geometry is merged with a matrix and meshed with reduced order voxel elements to create a representative volume element (RVE) of the braided carbon fiber reinforced polymer (CFRP). A linear elastic mesoscopic FE simulation using the developed RVE is performed for extraction of the orthotropic elastic constants, and excellent agreements with experimental data were obtained. 12
Thermo-Mechanical Characterization of a Hybrid Reinforced Photopolymer Composite via DLP 3D Printing
The application of 3D printing in diverse industries is continuously on the increase because of its enormous benefits. However one drawback of using this techniques is the weak mechanical parts it produce. Recent studies have included carbonnanotubes (CNTs) into printed parts to improve strength using the DLP 3D techniques. Increasing the CNTs in nanocomposites decreases the mechanical properties as the CNTs absorb the UV-light irradiation meant for polymerization. This research consider basalt fillers as an alternative and CNTs/basalt hybrid reinforcement for better performance. A 5-20ï­m long multiwall carbon nanotube of diameter 30nm and 10ï­m chopped basalt fiber (BA) of diameter 10-19ï­m were used as the reinforcements. A commercially available Phrozen nylon-like photoresin of viscosity 7mm2/s was used as the matrix. A Phrozen Shuffle 3D printer was used to print dog-bone specimens according to tensile test standard ASTM D638 for composites. Mono-fillers composites of 0.1 wt%CNT, 0.3wt%CNT and 0.1wt%BA, 0.3wt%BA were printed. Bi-fillers composites were printed by vary the CNTs from 0.05wt% to 0.2wt% and the Basalt fiber from 0.1wt% to 0.3wt%. Fillers were mixed with the resins by ultra-sonication method. Samples were characterized by performing mechanical tests with the MTS and decomposition thermal test was performed by the TGA. The Fourier transformation infra-red spectroscopy (FTIR) was used to evaluate double bond conversion and SEM instrument was used to characterize the fractured surface of each samples. Basalt mono-filler composite had better mechanical property than CNTs mono-filler composite. Increasing the wt% of CNTs in the hybrid composite decreases the tensile strength, elastic modulus and toughness while increasing wt% of basalt filler increases these properties. Composite samples with lower wt% of fillers had higher C=C conversion as UV-light could penetrate for greater level of polymerization resulting
Effect of Uniaxial Compression on the Shape Memory Behavior of Unidirectional Vitrimer Composite Embedded with Shape Memory Polymer Fibers
A major challenge polymer composites face is micro-cracking that results from various loading conditions. The affected structural members release energy in the form of micro-cracks, which could worsen with time and impair the functionality of the member. In recent studies, these propagating cracks are braced and closed by taking advantage of the shape-recovery property of shape memory fibers or shape memory matrix, before healing. For self-healing composites embedded with tensionprogrammed shape memory polymer fibers as sutures, the composites may be subjected to compression loading during service. Compression loading may reduce the stored strain in the embedded pre-stretched fibers, and thus reduce the shape recovery strain and limit the ability to close cracks. If the matrix is also a shape memory polymer, a compression load to the composite includes the matrix as well. Consequently the matrix would expand when the composite is heated thereby mitigating the shrinking ability of the pre-stretched fibers. The purpose of this study is to investigate the shape memory effect of the composite when pre-stretched shape memory fibers are embedded in a shape memory polymer matrix, in particular, the shape memory effect of the composite if the composite is subjected to compressive load during service. To this purpose, pre-stretched shape memory Polyethylene Terephthalate (PET) fibers with different fiber volume fractions were embedded into a shape memory vitrimer. The shape memory effect of both the PET fiber and the vitrimer was investigated. The effect of compression load on the shape memory behavior of the composite was studied. It is found that, uniaxial compression on the composite along the fiber direction significantly reduced the shrinking ability of the embedded pre-tensioned shape memory polymer fibers, and thus it is a factor that needs to be taken into account when designing such type of self-healing composites.
Evaluation of an Indirect Heating Method for Self-Healing in Shape Memory Fiber-Reinforced Polymer Composite Using High Intensity Focused Ultrasound
Materials researchers have made significant efforts to develop materials capable of mending cracks via the ingenious application of self-healing and shape memory properties. The development of self-healing and shape memory processes rely heavily on the trigger mechanisms for achieving them. Existing trigger mechanisms for self-healing often depend on the inclusion of foreign material, which is responsive to a particular stimulus of interest, such as electricity. These inclusions often negatively impact overall material composition, properties, and performance in diverse ways. In this study, effort is made to explore the potential for using High Intensity Focused Ultrasound (HIFU) as a trigger for shape memory effect and selfhealing of a fiber-reinforced thermosetting polymer composite. The objectives of the study were achieved via a Finite Element simulation in COMSOL Multiphysics. Simulation results showed that a temperature rise of up to 3.6K was obtained at the outer surface of the embedded fiber after 2 seconds with only 1 second of the insonation of an 8mm thick composite model while a temperature rise of up to 2.4K was obtained within the fibers. The onset of the heating was in the regions of highest power dissipation at the matrix-fiber interfaces along the sound propagation path. Subsequent heating of the embedded fibers was primarily via conventional thermal conduction from the interface into the fiber material. These results help provide a better understanding of the acoustic and thermal effects of ultrasound on fiberreinforced composites.
Sustainability of Glass Fiber Reinforced Self-healable Shape Memory Polymer Composite Laminates with Shape Memory Alloy Z-pins
Owing to their unique and outstanding in-plane properties and high specific strength and stiffness, fiber-reinforced polymer composite laminates are being used widely for many structural applications, such as aircrafts, infrastructure, and automobiles. Notwithstanding, they are normally susceptible and vulnerable to damage from out-of-plane impact events. Low velocity impact of fiber-reinforced composite laminates often results in damages that are invisible, but would progressively propagate and later results in a catastrophic failure. This study focused on developing a self-healing composite laminate with improved transverse strength and cyclic healing capabilities that would address the problem of delamination. A novel self-healable and recyclable vitrimer-based shape memory polymer (VSMP) was used as the matrix, unidirectional Saertex glass fiber as the reinforcing fibers, and tension programmed shape memory alloy (SMA) wires (Flexinol) as z-pins. This design followed the strategy of close-then-heal (CTH) for delamination healing. Low velocity impact tests, compression after impact tests, and self-healing of impact induced delamination were investigated. The tension programmed SMA z-pins not only helped resist delamination during impact, but also helped narrow/close the delamination through constrained shape recovery during heating, so that the narrowed/closed delamination can be healed repeatedly by the VSMP itself. The novel hybrid composite laminate provides a promising sustainable multifunctional material system for structural application.
Virtual Design and Demonstration of a Carbon Fiber Composite Load Floor
Despite the advantages in weight savings and parts consolidation of composites relative to steel and aluminum, their use for automotive structural components has traditionally been limited by the lack of reliable material and component modeling capabilities. Our team has developed models that allow us to better understand the multi-scale, multi-physics that govern the manufacturing process of composite materials so that we can simulate the manufacturing of continuous carbon fiberreinforced composite parts with much-improved precision. This paper details the design and manufacturing approach for a composite load bearing floor that is manufactured using high pressure resin transfer molding (HP-RTM). While resin transfer molding (RTM) is a relatively old technology that traditionally required cycle times that were far too long for most automotive applications, by injecting the resin at higher pressures, significant improvements are possible. Additionally, significantly faster-curing epoxy resin systems have become commercially available, enabling lower cycle times. Taken together, these developments have made HP-RTM a highly desirable molding process for volume manufacturing of complex parts. HP-RTM also easily allows for the use of continuous fibers, which are crucial for an assembly to meet rigorous crash performance requirements. The manufacturing process was modeled extensively to predict the optimal injection strategy and molding conditions. Feedback from these simulations was used to rationally develop the tooling design. This floor’s large size presented particular challenges for filling the part before the resin reached its gel point. The key variables
Computational Tool Development for Tailored Fiber Placement (TFP) Design Optimization
Tailored fiber placement (TFP) is a composite preform manufacturing technique that automatically lays down fiber bundles at specified locations, with the ability to provide out-of-plane reinforcement via localized stitching. This process promises to improve mechanical performance and reduce component weight by laying down fiber reinforcements in a strategic and geometrically optimized manner. Furthermore, TFP can be used to produce near net-shape components in an automated fashion using commercially available machinery. However, the degree of property improvement and weight reduction directly depends on the location of fiber bundles and reinforcing stitching. Currently this design process is iterated upon by costly and time-intensive trial-and-error experimentation. The present work seeks to streamline the design process by developing a suite of agile computational tools that can accurately model the mechanical performance of TFP preforms. This work presents the methodology for simulating the impact of reinforcing stitching on the performance of a component. A special technique is devised to create reinforced inter-ply interfaces into progressive damage analyses performed by finite element method (FEM) package BSAM to understand this impact and optimize the reinforcement for maximum performance. Two distinct TFP components were constructed utilizing T700S 12k fiber tow and infused using the epoxy resin RTM6. These components were then constructed and tested for comparison to the model.
Artificial Neural Network Approach to Tailor Composite Materials with Nonlinear Viscoelasticity
Material characterization is a major challenge for viscoelastic composite materials. Due to nonlinearity of materials properties, it is overwhelming to test each composite material for every application case under the combined effects of temperature and strain rate. Machine learning methods can help by using the existing datasets to predict properties over a different combination of parameters. This work focuses on building an artificial neural network (ANN) architecture to help in predicting properties and compositions of viscoelastic materials. The high density polyethylene (HDPE) syntactic foam is used as a case study material. Four types of HDPE syntactic foams were tested using dynamic mechanical analysis (DMA). Then, ANN was used to build the master relation of viscoelastic properties with respect to frequency, temperature, particle volume percentage and strain. The master curve for storage modulus was then transformed to time domain relaxation function and used to predict the stress-strain relations. The elastic modulus was extracted and compared to experimental results from tensile tests. The results show good agreements in properties of syntactic foams with both tested and extrapolated compositions. These results show that ANN can help in designing composite materials using machine learning methods on a limited dataset.
Methodology for Designing Bio-Like Structures with Diverse Geometry and Hierarchical Topologies
Biomimetic research has established that the key to the remarkable composite materials found in nature lies in a unique combination of constituent properties and hierarchical structural design. However, these design concepts have not been readily adapted to synthetic materials, because it is still unclear how to apply biolike principles to practical engineering applications. There is a need for a robust methodology that is capable of generating biomimetic structures with complex geometry and multiscale hierarchy, while accounting for application specific design criteria. The purpose of this work is to demonstrate a novel bio-like design methodology that meets these objectives. The approach is based on a bio-inspired pattern generation algorithm, the Gray-Scott model, which employs reactiondiffusion kinetics typically used to study morphogenesis. From a geometric perspective, the model is capable of producing a wide array of bio-like patterns such as spots, stripes, coral, and lace by controlling the reaction rates. For the first time, using this approach, we demonstrate the ability to produce hierarchical microstructures by controlling length scales using variable diffusion coefficients. The bio-like microstructures generated with this new approach complement recent advances in additive manufacturing that make fabrication of such novel structures feasible for the first time. This design paradigm permits the development of custom bio-like structures, thereby dramatically expanding the options for bio-inspired design of engineering materials.
Finite Element Analysis of 3D Woven Composites Using Consumer Graphical Processing Units
The focus of this paper is application of a Graphical Processing Unit (GPU) based solver to linearly elastic finite element analysis (FEA) of composites with threedimensional (3D) woven reinforcements. Aspects specific to this material system including local material orientations, high contrast between elastic properties of constituents, large number of degrees of freedom, and simulation runtimes are discussed. Speedups offered by parallelization via GPUs and regularity of structured grids enable matrix-free implementation of FEA, which requires reassembly of the global stiffness at every iteration of solution of the system of linear equations, but in turn significantly reduces memory requirements. This makes linear analysis of composite structures with explicit reinforcement representation (tens of millions of degrees of freedom) possible on personal computers. Potential applications of this procedure include fast calculation of effective properties for design of novel 3D woven architectures and efficient solution of problems with high degrees of material nonlinearity requiring frequent stiffness matrix updates.
Applications of Digital Element Approach in Textile Mechanics and Textile Composite Mechanics
The digital element approach (DEA) was originally developed to determine the geometry of textile composite preforms. With DEA, a fabric is considered as an assembly of unit cells; a unit cell as an assembly of yarns; and a yarn as an assembly of digital filaments. Each digital filament is discretized into a chain of digital elements. Contact force between digital filaments is calculated based on distance between them. During the past ten years, software, DFMA, has been developed based on DEA to simulate fabric deformation under external force in several areas of textile composite mechanics: determination of fabric microgeometry, simulation of fabric deformation in composite manufacturing processes, such as draping, forming and molding, and generation of conformal FEM meshes for composites with complex shapes. In this paper, principles of DEA are examined first. Applications of DFMA in the following four areas are then reviewed: 1) determine geometries of textile composite preforms with complex shapes, 2) generate conformal FE meshes for textile composites, 3) determine strength and shear characteristics of textile composite preforms and 4) simulate draping, forming and molding processes of textile composite preforms. Multiple examples are presented in detail to illustrate applications from each of the above four areas.
Implementation of Numerical Models to Evaluate Intrinsic Residual Stresses in 3D Woven Composites by Blind Hole Drilling Tests
3D woven carbon/epoxy composites are often produced using resin transfer molding technique which includes epoxy curing at elevated temperatures. The process may lead to accumulation of the intrinsic residual stresses during cooling of the material caused by the mismatch between carbon and epoxy coefficients of thermal expansion. This paper deals with implementation of mesoscale finite element models to evaluate intrinsic residual stresses in 3D woven composites. The stresses are determined by correlation of the surface displacements observed after drilling 1-mm diameter blind holes with the corresponding predictions of the models. We investigated how a numerical representation of the composite plate surface affects the correlation between the experimental measurements and numerical predictions and how it influences the evaluation of the process-induced residual stresses. It has been shown for ply-to-ply woven composites with different pick spacing that the absence of the resin layer leads to more accurate interpretation of the experimental measurements. The prediction of the average residual stress in the matrix phase of the composite was found to be sensitive to the surface representation accuracy, however, the residual stress magnitude and distribution was not affected fundamentally.
Mesh Convergence Study for the Mesoscopic Analysis of a Triaxial Braided Carbon Fiber Reinforced Polymer Under Uniaxial and Shear Loading States
In this study, the mechanical behavior and convergence of a finite element model of a carbon-fiber reinforced polymer (CFRP) composite with different mesh sizes are analyzed. A representative volume element (RVE) of a single-ply triaxially braided carbon fiber-epoxy composite is developed for this study. The model is subsequently meshed with periodic boundary conditions using the voxel meshing method. Numerical simulations of these models subject to uniaxial tension and shear are conducted using a commercial finite element (FE) code, Abaqus, to determine the in-plane mechanical properties of the polymer composite. To examine the corresponding convergence behavior, various mesh sizes are employed in RVE models generated. The models are compared based on their computational efficiency at a converged mesh size. It was determined that a medium sized domain length with an element length of approximately 0.1 mm is adequate to reach a converged solution with the shortest simulation time. The work provides a systematic mesh convergence examination of a popular meshing technique used in finite element analysis, which elucidates the optimal method to analyze this quasi-isotropic material in a streamlined fashion and paves an understanding of the advantages and limitations of this modeling approach. This analysis can be extended in future studies as a larger mesh convergence study that compares meshes with different geometric elements, such as 10-node tetrahedral element. The results of this study can also be used to simulate a larger continuum-sized composite material, in which computational efficiency is crucial.
Application of Molecular Dynamics and Micromechanics to Predict the Mechanical Properties of Boron Nitride-Reinforced Epoxy Composites
A multiple length scale approach was developed to investigate the feasibility of improving epoxy composites using boron nitride nanoplatelets. Molecular dynamic was utilized at the lower length scale to construct and characterize the mechanical properties of the microstructures, while micromechanical theory was applied to the modeling and simulation of the material systems at higher length scale to quantify their elastic properties at various loading conditions and relative fractions of micro constituents. In this research study, the techniques for evaluating the mechanical loadings were also defined. The predicted results of the analysis showed that increasing the h-BN concentration increased the tensile and shear moduli of the nanocomposites due to higher rigidity of the boron nitride nanoplatelets embedded in epoxy.
Multiscale Failure Analysis of a 3D Woven Unit Cell Containing Defects
Multiscale failure simulations have been performed for a Three-dimensional woven composite unit cell considering five, or more, length scales spanning the woven composite mesoscale to the submicroscale voids. The multiscale recursive micromechanics approach, which enables recursive integration of general micromechanics theories over an arbitrary number of length scales, has been employed within the NASA Multiscale Analysis Tool. The multiscale model uses both the generalized method of cells and Mori-Tanaka micromechanics theories, and considers failure in the constituent materials using a simple damage model. Baseline results, containing distributed voids, are compared to uniaxial experimental data for an AS4 carbon fiber/ RTM6 epoxy matrix 3D orthogonal woven composite with good agreement in terms of global stiffness and global failure stress. The simulations demonstrate that the 3D woven composite exhibits damage tolerance through sustaining increasing axial load far beyond the first initiation of damage. The multiscale model is used to examine the nonlinear response of the material to other loading conditions. Case studies, motivated by X-ray computed tomography data, are presented on the effects of manufacturing induced voids and cracks.
Improved Plate and Beam Models for Thermoviscoelastic Constitutive Modeling of Composites
The effective properties of composites are influenced by the time-dependent behavior of polymer matrices, which are very sensitive to changes in temperature. Improved plate and beam models are required to efficiently design, and simulate composite structures when the long-term performance of large anisotropic composite structures is the matter of interest. In this work, mechanics of structure genome (MSG) is used to construct linear thermoviscoelastic plate and beam models that can homogenize three-dimensional heterogeneous materials made of constituents with time- and temperature-dependent behavior. The formulation derives the transient strain energy based on integral formulation for thermorheologically simple materials subject to finite temperature changes with the restriction that the strain is small. The reduced time parameter is introduced to relate the time-temperature dependency of the anisotropic material by means of master curves at reference conditions. The new formulation has been implemented in SwiftCompTM, a general-purpose multiscale constitutive modeling code based on MSG. Experimental data and three-dimensional direct numerical simulations of thin-ply high-strain composites (TP-HSC) using a commercial finite element analysis (FEA) package are conducted to verify the accuracy of SwiftCompTM results. The paper also analyzes the relationship between the shift factor of the polymer matrix and the temperature dependencies of the effective beam properties.
Structural and Aeroelastic Evaluations of Wings with Lattice-Based Mechanical Metamaterials
Mechanical metamaterials are artificially manufactured materials with properties, which cannot be obtained from natural materials, consisting of deliberately designed microscopic internal structures. Those conceptual artificial materials are ready to fabricate with the advent of the additive manufacturing technology. Recently, a class of metamaterials has been actively studied due to its potentials for performance improvements and extensions of structural capabilities. Mechanical metamaterials can exhibit exotic material characteristics or macroscopic behaviors attributed to their micro/mesoscale internal structural designs such as truss or porous in μm/mm order in addition to material properties constructing the structural components. In this paper, structural and aeroelastic characteristics of wings integrated with lattice-based mechanical metamaterials are studied. In order to efficiently model such wings, a computational homogenization method for lattice-based mechanical metamaterials capturing characteristics of a representative unit cell is implemented. By assuming periodic distributions of the mechanical metamaterials in the wing structure, a unit cell of the periodically distributed structure is modelled as representative finite elements. The equivalent stiffness properties of the unit cell are calculated based on a computational homogenization procedure. The obtained equivalent stiffness properties are used to perform a multi-scale structural and aeroelastic simulations of a wing with the lattice-based mechanical metamaterials by modelling the mechanical metamaterials as equivalent solid substructures. In the current study, simple rectangular thin wings integrated with the lattice-based substructures are designed, and the structural/aeroelastic performance of the wings are also explored using the multiscale analysis method to demonstrate the feasibility and potentials of such wings.
Investigation of the Mechanical Properties of TPU on Molecular and Micro Scale
The mechanical properties of thermoplastic polyurethane (TPU) are highly dependent on the chemical composition [1]. This dependency was investigated with micromechanics based and molecular dynamics simulations. TPU is a copolymer which consists of hard and soft segments. The microstructure of the copolymer for different mass fractions of the hard segment was determined with the group contribution method [2] and the phase diagram according to [3]. The results are compared with the microstructures obtained with the Self Consistent Field Theory [4]. For the assessment of the Young’s moduli of the segments tensile tests on the molecular level were performed. Furthermore, a sensitivity study of several parameters which have an influence on the results of the molecular dynamics simulations was performed. With the information about the Young’s moduli and the microstructure micromechanics based simulations were performed to determine the Young’s modulus of the TPU. The outcome of this research study is a complete workflow for the determination of the elastic properties of multiphase materials based on the molecular structure of the phases using molecular dynamics and micromechanics simulations. Multiphase ma- terials, so called digital materials, can be manufactured for example with the polyjet 3D-printing technology. Thus, a future application of the developed methodology is the investigation of the elastic properties of 3D-printed digital materials.
Compression Behaviour of Pseudo-Ductile Carbon/Glass Hybrid Laminates Pre-Fractured in Tension
Specimens with a pseudo-ductile high strength carbon / S-glass hybrid sublaminate near one surface were loaded in bending, producing various degrees of fragmentation in tension. They were then turned over and loaded in compression, showing a substantial reduction in residual compressive strain due to early delamination, but the degree of pre-fragmentation did not have a pronounced effect.
Effects of Graphene Nanoplatelets Dispersion on the Enhancement of Tensile Strength, Thermal and Electrical Conductivity of Polymer Nano-Composites
The characterization of mechanical, thermal and electrical conductivity of samples containing homogenous mixtures of Epoxy matrix diglycidyl ether of bisphenol A (DGEBA) and varying amount of graphene using solution compounding method coupled with high shear mixing were carried out in this study. The measurement of tensile strength, thermal and electrical conductivity of the produced samples at 24oC (ambient) temperature was carried out using the universal tensile, A lasercomp Fox 50 and Keithlev 8009 electrometer testing machine in accordance to ASTM E8/E8M-13M, C518/ISO 83011 and ASTM D257-14 respectively. The magnitude of the tensile strength, thermal and electrical conductivity increased evidently with an increase in percentage weight fraction of the GNPs. The fractured surfaces of the tensile strength test samples were further examined by scanning electron microscopy. There is an increase in the tensile strength, thermal and electrical conductivity of epoxy-graphene nano-composites post cured with microwave compared to samples post cured with conventional autoclave oven as well as cast neat epoxy. The use of solution compounding coupled with high shear mixing as dispersion technique influences the homogeneity and mechanical properties of the nano-composites positively. It is concluded that epoxy-graphene nanoplatelets nanocomposites possess better qualities in terms of tensile strength, thermal and electrical conductivity for light weight and high strength industrial applications. This work find application in the automobile, electronics and aerospace industries in their quest for sustainable material development.
Reinforcement of Silanized Milled Graphite Nanoparticles to Enhance Mechanical and Fracture Performance of Epoxy Nanocomposite
In this work, we used milled graphite nanoparticles (GrNPs) to fabricate highperformance epoxy nanocomposite. Motivation aroused due to the ease of production of GrNPs from graphite flakes and their cost-effectiveness. Herein, The GrNPs was silanization using 3-aminopropyltriethoxysilane after oxidation. The oxidation of GrNPs helps to disaggregate and generate oxygenated functional groups at their surfaces. These oxygenated functional groups were utilized to graft active amino functional groups at GrNPs. FTIR and TGA analysis are conducted to confirm the successful silanization, whereas, FESEM and TEM are used to study the morphology of GrNPs. Morphological analysis of GrNPs reveals its porous structure consisting of stacked and randomly oriented planes with a large number of defect sites. All these pores and defects are found well camouflaged after silanization, as can be seen from FESEM morphology analysis in Figure 1. Both pristine and silanized GrNPs (0.5 wt%) were dispersed separately into the epoxy resin system and cured. The testing results reveal that the addition of 0.5 wt% of pristine GNPs in the epoxy system enhances the tensile strength, Young’s modulus and fracture toughness by ∼2%, ∼- 7%, and ∼-16% respectively with respect to that of neat epoxy. Whereas, the formation of active amino functional groups on silanized GrNPs helps them to covalently bond with epoxy network and hence increased the same properties by ~33%, ~5%, and ~43% respectively. Such enhancement in the mechanical and fracture properties can open up the scope for the utilization of GrNPs as a potentially cost-effective and alternative candidate to mitigate damages of epoxy nanocomposites compared to CNTs as well as graphene.
Micromechanical Modeming of Hybrid Glass Fiber Laminated Composites Added with Graphene Nano Platelets
The standard engineering materials are being fast replaced by composite materials due to their superior strength to weight ratio, ability to be custom-tailored, and their remarkable stiffness characteristics. Due to the above mentioned positive traits, these novel materials are finding their application in every wakes of life and are no more considered as a space-age material. In the last decade, composites with nanofillers infused matrix are a major research topic due to its potential to overcome the inherent weakness of laminated composite materials like weak intralaminar fracture toughness, tendency to delaminate, etc. Among the different types of carbon-based nanofillers, graphene is the most novel one. As such, the requirement for prediction of the properties of graphene-based laminated composites has arisen. In the present study, an attempt has been made to predict the stiffness matrix of the graphene nanoplatelet infused glass fiber reinforced laminated composites using micromechanical methods. Provision for accommodating the agglomeration and property variation of graphene is made in the program for calculating the stiffness. The mathematical modeling confirms that the agglomeration of nanoplatelets drastically reduces the effective properties of GNP infused laminated composite. The proposed model also shows that a variation in the platelet size can cause significant variation in Young’s Modulus and Poisson’s Ratio. The weight fraction’s under consideration in this study, i.e. 0.5% and 1.0% of GNP improves the tensile properties, interlaminar and intralaminar properties of glass fiber laminated composites. The addition of nanoplatelets increases the stiffness of the GNP infused laminates. A reduction in platelet size of graphene can give better results for laminated composites as the filtering effect of the glass fiber will be reduced and a more even distribution of graphene in the epoxy can be expected. The specimen cutting method and machine error might have caused the significant variation in the predicted and experimental results.
Modeling the Effect of Nanoparticle Size on Fracture Toughness of Nanocomposites
The objective of this paper is to investigate the validity of application of continuum-based linear elastic fracture mechanics (LEFM) methodology, which is often employed by researchers to model fracture processes at the “discreteâ€ atomic scale. The material selected for this study is EPON 862 epoxy polymer with 85% cross-link density. Further, an atomistic J-integral is implemented as a nano-scale fracture metric to investigate flaw-tolerance at the nanoscale reported by many researchers, and to develop a methodology to predict the initiation fracture toughness of the material. For this purpose, a bond-order based potential (ReaxFF) available in LAMMPS molecular dynamics (MD) software is utilized. Predictions obtained using the atomistic J-integral are compared with LEFM predictions for the case of a crosslinked epoxy polymer block with a center-crack under uniform far-field loading. Significant deviations from LEFM for crack-lengths below a certain critical cracklength threshold are observed. Further, far-field stress-strain plots are obtained for a center-cracked epoxy polymer block with a graphene nanoplatelet embedded ahead of the crack tip and compared with stress-strain plot obtained for neat baseline epoxy
Molecular Dynamics Study of Interfacial Properties in Flattened Carbon Nanotubes and Polymer Composites
In this study, the interaction characteristics of two polymer systems (fluorinated and non-fluorinated polyimides) with flattened carbon nanotube (flCNT) surfaces are predicted with molecular dynamics simulation and directly compared. The results show that the presence of the CF3 groups in the fluorinated polyimide creates increased steric hindrance at the interface which causes a reduction in the interaction energy with the flCNT surface and an increase in the frictional force resistance of the flCNT to shearing motion.
Structural Evaluation of Fly Ash Based Geopolymer Composites for High Temperature Applications
The current research work focuses on the enrichment of fly ashes to the geo-polymer composite for high temperature application to 800ËšC-1200ËšC. The goal of the research is to enrich fly ash in order to obtain a pure natural alumino-silicate pozzolan. Mild acid and thermal treatment of different grades of fly ashes is carried out followed by geo-polymerization in the presence of potassium hydroxide environment. The process is further refined with the addition of the fumed silica to reduce the process related porosities which can deteriorate the overall performance of the geo-polymer. Continuous fibers are added to study and evaluate the interfacial bonding. The flexural tests result of the different samples cured at different temperatures were analyzed to better understand the flexure strength of the geopolymer composite fabricated using glass fiber. Dynamic Mechanical Analysis (DMA) of the geopolymer was performed to evaluate its ability to withstand high frequencies and vibrations. Load Deflection patterns suggest that consolidation process continues as the temperature increases from 80 °C to 250 °C, which is reflected by the stiff load-deflection curves. As the temperature increases to 1200 °C sample gradually fails suggesting inter-laminar delamination. DMA results suggest that storage modulus increases with temperature rise upto 350 °C at or below 100 Hz frequency. Fire Test did not result any structural deformation or blistering on either the front or back face.
Hierarchically Organized Nano-modified Carbon Fiber Composites for Enhanced Fatigue Life of Structural Components
Carbon fiber reinforced polymer composites (CFRP) are ubiquitous in engineering applications where they are employed as structural components. Fatigue failure which originates in matrix of CFRP is a major design constraint in these composites. In this work we demonstrate considerable increase in fatigue life in nano modified carbon fiber reinforced polymer composite (nanoCFRP) through addition of nanoscale silica particles. Fumed silica particles are used as nanofillers which are incorporated in epoxy initially to explore their effect on fracture and fatigue performance. The results indicate improvement uniform improvement in the fatigue life with increasing silica loading fraction. Fractographic analysis performed indicated presence of various toughening mechanisms like crack deflection, void growth and heterogeneity induced toughening. The epoxy-silica nanocomposite was then used as the matrix material with carbon fibers to fabricate nanoCFRP with different silica loading fractions. nanoCFRP samples were initially tested statically in three point bending setup and the static strength was found to be uncorrelated with the loading fraction. The cyclic flexural tests, however, revealed significant improvement in fatigue life in nanoCFRP composites. The fatigue life improvement was pronounced in high cycle fatigue regime.
Wetting and Spreading Kinetics of a Nano-suspension Drop via MD Simulations
Nano-suspension has garnered tremendous attention recently in advanced material processing. The concept of assembling ordered arrays of nanoparticles on a substrate surface via suspension droplet wetting and subsequent evaporation has fueled a large body of research in this area. Wetting and Spreading kinetics of nanosuspension droplets are discussed by results from Molecular Dynamics Simulations. We examine how wetting kinetics depend on the effects of nanoparticle size and interaction strength. Results demonstrate that increasing the particle size dramatically changes observed wetting behavior from depinning to pinning. Additionally, simulation results show how weakening the interaction between the particle and the underlying substrate can change a system from exhibiting pinning behavior to de-pinning.
A Study of the Effect of Diameter, Chirality and Defects on the Energetic Stability of Carbon Nanotube Junctions
Carbon nanotube junction (CNTJ) induces discontinuities (defects) on the regular hexagonal atom arrays of single-walled carbon nanotube (SWCNT) during junction formation. The degree of discontinuity depends on the nature (i.e. chirality) of participating SWCNT structures forming the junction, which eventually affects the stability/integrity of the junctions and its functional properties. The competing parameters on the energy stability of SWCNT junctions are diameter, chirality, and set of defects. Here in this work, the effect of each parameter on the energetic stability of SWCNT junction is discussed, in which SWCNT junctions are created using the systematic approach developed by Varshney et al1. Heptagons and pentagons of carbon atoms are found to be the most occurring defects on the lowest energy SWCNT-junction structures. Among the three parameters studied, the diameter is found to be the dominating parameter which determines the energetic stability of the junction.
Molecular Modeling of Glass-Epoxy Interphase: Influence of Chemistry and Molecular Weight of Silanes
In this paper, we study the effect of chemistry and molecular weight (MW) of silanes on the glass fiber-epoxy interaction using all-atom molecular dynamics (MD) simulations. Silane with two different end chemistries (Amino and Glycidoxy) and four MWs (Aminopropyl trimethoxysilane (APS, 179 g/mol), 3- Glycidoxypropyl trimethoxysilane (GPS, 236 g/mol), 8-Glycidoxyoctyl Trimethoxysilane (GOS, 306 g/mol) and 16-Glycidoxyhexadecyl Trimethoxysilane (GHS, 418 g/mol)) are considered. For the matrix, Diglycidyl Ether of Bisphenol A (DGEBA) and Jeffamineïƒ¢230 curing agent are considered. In the presence of mono-layer silanes, the interphase forms between glass fiber and epoxy matrix. The silane layer has a 0.98nm-2 silane number density, which covers 25% of the glass fiber reactive sties. The fiber-matrix composite system with mono-layer silane is created through condensation and curing reaction using the general AMBER force field (GAFF). The composite system is then subjected to transverse tensile and shear loadings using reactive force field ReaxFF to predict mechanical properties and damage mechanisms. The interphase thickness is predicted to be in the range of 1.1-1.4nm. Within the interphase various connectivity patterns are identified for the amine and epoxide terminated silanes. The interphase strength exceeds the strength of the bulk epoxy, leading to cohesive failure in the matrix for all silane types investigated.
Nanocomposites Membranes for Acoustics Applications: A Virtual Noise Free Airplane Cabin
The association in series of aerogel and nanomembranes seems to be effective for acoustic applications. The addition of carbon nanotubes to PVDF-TrFe solutions not only led to changes on fiber diameter but it also affected fiber’s spatial distribution. Such changes on fibers’ spatial distribution could be the main factor for variations on air flow resistivity. The best results, at 500-1500 Hz range, were obtained by the group with the smallest airflow resistivity. A possible explanation for such behavior could be that the air flow induced vibration of the nanomembrane itself. For aerospace applications, in special airplane cabins, the aero-nanomembranes composite seems to be a valuable solution as it can block up to 80 % of noise at 1500 Hz frequency range with a 60 mm thickness.
Fabrication of Submicron Thermosetting Carbon Nanotube-Epoxy Fibers Using Electrospinning
Recently epoxy-based nanocomposites are gaining tremendous attention in many structural applications such as those in aerospace, automotive and motorsports. This research represents a new approach to fabricate submicron thermoset epoxy filaments enhanced with carbon nanotubes (CNT), through optimized curing followed by an electrospinning process. The optimized curing process is based on the uniform mixing of CNT with epoxy, and partial curing of the CNT/epoxy mixture with the hardener through a thermal treatment without adding any plasticizers or thermoplastic binders. Later the fibers have been made by electrospinning of the semi-cured mixture. Fig 1 shows the fabrication process of the described filaments. The key goal is to make the thermosetting epoxy without adding any thermoplastic to keep the integrity and quality of the fibers. The diameters of these filaments can be tuned between 100 nm to 500nm. Further, the CNT structure has been aligned inside the filament structure by the presence of the electrostatic field during the electrospinning process results in better stability and smaller diameters for the fibers. The fabricated filaments show that adding a low amount of CNT in the epoxy structure, better structural, electrical and thermal stability, has been achieved.
A Multiscale Framework for the Progressive Damage Analysis of Fiber-Reinforced Composites Using a Component-Wise Approach
The objective of the current work is the development of a multiscale numerical platform for the analysis of fibre-reinforced composites, using higher-order structural theories based on the Carrera Unified Formulation (CUF). The present work extends previous works on CUF multiscale modelling by considering an explicit time integration scheme for the macroscale analysis. Each integration point at the macroscale is informed by the micromechanical analysis of a representative volume element (RVE). Nonlinearity in the analysis arises from nonlinear material behaviour within the matrix constituent of the RVE. Composite inelasticity is modelled using the von Mises plasticity theory, while progressive damage is modelled using the crack band approach. Numerical assessments are performed to predict composite inelasticity via the explicit multiscale approach, and the results are found to be in good agreement with experimental data and reference numerical results. Structural-level multiscale progressive damage analysis is also performed to demonstrate the capability of the multiscale framework, and the obtained results are in good agreement with reference numerical results from the literature, thus verifying the framework for progressive damage analysis.
Evaluation of Notched Strength of Thin-Ply Composite Materials Using Adaptive Discrete-Smeared Crack (A-DiSC) Method
This study develops a multi-fidelity fracture simulation scheme of composite materials which combines the Adaptive Discrete-Smeared Crack (A-DiSC) method and the theoretical model for saturated crack density. The A-DiSC employs the discrete crack model (DCM) firstly to model the transverse cracks in the laminates. If the crack density of each ply reaches the saturation value calculated by the theoretical model, the DCM is converted to the smeared crack model (SCM). At this point, some cracks that are longer than a certain critical length remain explicitly modelled by DCM since they can significantly affect subsequent path to final failure and fracture. By combining the two models, this method reduces the computational cost while preserving sufficient prediction accuracy. In order to verify the prediction accuracy of developed scheme, the simulation of the open-hole tensile test of thin-ply composite laminates were carried out, and predicted results were compared with the experiments.
Numerical and Experimental Evaluation of the Effects of Ply Thickness on Progressive Damage in CFRP Laminates
The purpose of this study is to evaluate the effects of ply thickness on the progressive damage in thin-ply composite laminates experimentally and numerically. To investigate the influence of the ply thickness on the damage growth, loading and unloading tests were carried out on angle-ply and quasi-isotropic laminates with three different ply thicknesses. The damage evolution was evaluated quantitatively using continuum damage mechanics model. The evolution laws of damage in thin-ply composites were formulated from the experimental data considering the effects of ply thickness. A damage simulation combining finite element analysis and the proposed damage model was conducted to predict the damage progress and strength of thin-ply laminates with several ply thicknesses. The simulation results were compared with the experimental data for the validation of the simulation method.
Validation of Delamination Growth Predictions under Fatigue Loading using the Single Leg Bending Specimen
The Single Leg Bending Specimen (SLB) was chosen for a study to compare experimentally determined delamination growth with analysis predictions. First, fatigue tests were performed on SLB specimens with several different starter delamination lengths. Specimens were cycled in load control and delamination length increase was continuously recorded with increasing number of cycles. Test results from different load levels were combined to capture the entire delamination growth law (Paris Law) over a wide range of energy release rates. Second, a recently developed interpolation scheme was used to predict the Paris Law associated with the SLB specimen. This interpolation scheme allows the calculation of the growth rates for a given mixed-mode ratio from known single-mode growth laws. Third, finite element analyses were performed using two-dimensional models of the SLB specimen. New Virtual Crack Closure Technique-based analysis tools for automated delamination growth analysis under cyclic loading in Abaqus were used for growth predictions. Fourth, the predictions from analyses were compared to experimental fatigue data. Remaining analysis shortcomings were highlighted
Effect of the Shape of the Softening Damage Law on the Predicted Fracturing Behavior of Composites
The fracturing behavior of fiber-reinforced composites is often modeled using continuum damage mechanics-based approaches which commonly assume linear softening, i.e. a linearly decreasing stress with increasing damage. The objective of this paper is to analyze the effect of the assumed shape of the softening damage law at material point, on the predicted fracturing behavior of the structure. The material considered is an epoxy/carbon-fiber twill woven composite. Numerical evaluations are conducted by predicting important phenomena associated with fracturing viz. the strength size effect and progressive accumulation of damage in a structure. Damage models with a wide range of shapes of softening laws are implemented as user-defined subroutines (VUMAT) in the commercial finite element analysis software ABAQUS. These shapes include linear, bilinear, trilinear, exponential, power law and sigmoidal. All the modeling is conducted via the crack band model (CBM), to ensure the mesh objectivity of results. It is found that despite the same strength and fracture energy used as input at the material point level, the structure level predictions can differ considerably. It is found that not all softening law shapes accurately capture the strength size effect and the post-peak softening behavior. The bilinear softening law shows the best combination of simplicity and accuracy, especially if formulated via the R-curve approach. Even for the bilinear shape, the precise position of the “knee pointâ€ is shown to be important. It is thus demonstrated that in addition to material strength and fracture energy, it is important to correctly formulate the shape of the softening law for accurate predictions of structural failure.
An Abaqus Implementation of the Regularized Extended Finite Element Method with Novel Mixed Mode Cohesive Fatigue Model
The Regularized eXtended Finite Element Method (Rx-FEM) is a fracture mechanics based technique for progressive failure simulation in which multiple damage events, such as matrix cracks and delaminations, are introduced into the finite element model via mesh-independent displacement discontinuities. In the Rx-FEM, the Heaviside step function used in the eXtended Finite Element Method (x-FEM) is replaced by a continuous function approximated by the FE shape functions. This regularization lends itself to a natural implementation in commercial finite element software as a superposition of native elements. The advantages of Rx-FEM implementation lie in its ability to utilize built-in capabilities from the host software, including contact, variety of native elements, geometric nonlinearity, as well as the standard post-processing, and visualization tools. This capability of Rx-FEM to insert mesh-independent cohesive interfaces also makes the method particularly well suited for the analysis of fatigue damage propagation in composites. A recently-developed cohesive fatigue model was implemented into the Rx-FEM framework for the Abaqus software suite. This fatigue model assumes that the cohesive law that describes quasi-static tearing is the envelope of the fatigue damage. Fatigue damage accumulates within the envelope at a rate that satisfies the S-N diagram and Miner’s rule. The parameters of the model are obtained from idealizations of S-N diagrams used in engineering design. The fatigue model relies on intrinsic relationships between S-N curves and their corresponding Paris law curves to predict the rates of crack propagation. The implementation of the new fatigue model is demonstrated by comparing the experimental and predicted response and failure of a mixed-mode bending (MMB) test, as well as a Clamped Tapered Beam (CTB) sub-element. The CTB is a test designed to study matrix crack and delamination initiation and subsequent propagation and migration from one ply interface to another.
Effects of In-situ Stress State on the Plastic Deformation, Fracture, and Size Scaling of Thermoset Polymers and Related Fiber-reinforced Composites
This work proposes an investigation on the effects of in-situ stress state on the plastic deformation, fracture, and size scaling of thermoset polymers and their fiberreinforced composites. Towards this end, a comprehensive set of size effect tests on pristine epoxy is presented along with a computational analysis within the framework of Drucker-Prager (DP) plasticity. This analysis includes the following experimental tests on geometrically-scaled specimens representing various loading conditions: (1) uni-axial tension; (2) three-point bending; (3) Mode I fracture; (4) Mode II fracture. The foregoing results clearly show that the mechanical behavior of thermoset polymers is strongly affected by the local stress state since the material at the notch tip can exhibit remarkable ductility with about 70% equivalent plastic strain in Mode II fracture due to deviatoric stresses whereas the pronounced brittleness characterizes the material at the crack tip with about maximum 1% counterpart under Mode I scenario due to hydrostatic stresses. In addition, thermoset polymers also exhibit distinct mechanical behaviors at various length scales since the material strength at the micro scale can be four to six times higher than the macro-scale value from conventional tests whereas the fracture energy at the micro-scale can be forty times lower. These interesting conclusions were further confirmed through the successful microscale damage modeling of crossply [90/0]s and [0/90]s laminates under uni-axial tension and pure shear by leveraging two-scale constitutive model. In fact, the undesirable micro-scale simulations by only considering the macro-scale material properties of thermoset polymers clearly convey the important information of tensorial stresses and multi-scale material properties in fiber-reinforced polymer composites.
Analytical Prediction of Mechanical Performance of Structural Organic Matrix Composites with Sharp Radii Features
An analytical approach for progressive damage prediction was developed for structural organic matrix composites (OMCs) with sharp radii features. Structural OMCs offer advantages in reduced weight and high mechanical performance. However, OMC structures with complex part geometries encounter frequent occurrence of defects due to multi-step reactions and void formation during manufacturing process cycles. Defect-driven analysis is particularly important for OMCs with sharp radii features. Porosity plays a significant role in the generation of microcracking and leads to the degradation of effective mechanical properties and fatigue life of engineering applications. In this work, a custom ANSYS® user subroutine was used to model the damage progression in canted-T shaped subelements with various porosity levels. The porosity was predicted based on incomplete step reaction using a composite process model. The relationship of property reduction as a function of porosity was established and quantified. The progressive damage finite element model, based on a custom algorithm that accounted for shear stress-strain nonlinearity, predicted different failure modes using both strength based and fracture toughness based failure criteria. Failure mode based stiffness loss, implemented at the element level using a nonlinear constitutive law, accounted for the current damage state. This model was used to perform a series of mechanical failure analyses to simulate laboratory test conditions. The predicted results showed a good correlation with experimental results for the entire nonlinear load deflection response, as well as the damage progression and local failure modes as observed by digital image correlation (DIC). Also studied and analyzed was the effect of local porosity on the damage initiation and progression at the sharp radii of the canted-T subelement using this custom ANSYS® model.
An Automated Meshing Framework for Progressive Damage Analyss of Fabrics Using CompDam
A meshing and model construction framework for progressive damage analysis of fabric composites is proposed. A mesoscale idealization is used such that the fiber tows are considered homogeneous and transversely isotropic continua, and the matrix is a homogeneous isotropic continuum. By using this idealization, relatively mature damage modeling techniques developed for laminates of unidirectional plies are transferrable to fabrics. Cohesive interface elements and a continuum damage mechanics (CDM) model implemented in the software CompDam are used to account for the key damage modes including cracking at the tow/matrix interface, transverse matrix cracking in the tows, and fiber breakage in the tows. Matrix degradation is accounted for by plasticity. The modeling approach is demonstrated for a 3D orthogonal woven fabric. The main novelty of the proposed model construction framework is in its flexibility resulting from a formulation that is agnostic to the fabric architecture and implementation using mesh based operations. The algorithm does not make any assumptions about the tow sizes, shape, path, or proximity to other tows, thus it is applicable to a broad range of fabric architectures. The matrix region mesh surrounding the tows is constructed from domain boundary and tow surface meshes (i.e. through a series of mesh-based operations) such that the model domain can be arbitrarily shaped.
Sensitivity of Crack Formation in Fiber-Reinforced Composites to Microstructural Geometry and Interfacial Properties
Cracks mostly happen at the fiber/matrix interface and then propagate through the composites. Full cracks can accumulate, induce delamination, and lead to failure of the composite structure. In this work, we use interface-enriched generalized finite element method scheme to simulate two-dimensional microstructural representations of carbon fiber-reinforced polymer matrix composite. Fibers, matrix, and fiber/matrix interfaces are modeled by elastic, elasto-plastic damage, and cohesive zone constitutive equations, respectively. The constitutive equations are implemented in a C++/MPI framework and designed to be memory efficient and able to run the simulations entirely in parallel. The developed framework is used to investigate microstructural representations of fiber-reinforced composites efficiently. The sensitivity of the crack’s formation pattern and strain level to geometrical and material properties of the microstructural representation are investigated. Nine different microstructural representations of the composite considering matrix with linear and nonlinear behavior and a range of interfacial strengths are studied. It was found that for high interface strengths, the nonlinear matrix can have an influence on the crack’s location. The outcome demonstrated that for lower interface strengths, the matrix nonlinear behavior do not affect the cracks’ development pattern
Real Time Identification of Interfacial Delamination in Sandwich Composite Through Point Sensing: Experimental Finding on Crack Propagation Morphology
Real time delamination propagation profile within the interface between the skin and polymer foam core of sandwich panels has been experimentally mapped with fiber optic Bragg grating (FBG) sensors embedded within the interface of pre-cracked double cantilever beam (DCB) specimens subjected to mode I dominated loading. Load-deformation is recorded from universal testing machine (UTM), crack-front propagation visible on the side of DCB is recorded from a camera attachment and the real time spectral data of sensors with wave length division multiplexing are recorded by interrogator. The crack propagation profile is mapped from the shifts in peak wavelengths of FBGs at different locations. The real-time crack propagation profile is mapped from the positions of crack-tips in parallel channels of FBGs. The results indicate non-rectangular non-parabolic profile and raises question on assumptions of numerical simulations for crack propagation problems of these types of sandwich composites.
Evaluation of Direct-Ink-Writing (DIW) of PDMS Hybrid Nanocomposite Ink for Piezoresistive Sensor Applications
Direct-Ink-Writing (DIW) provides unprecedented flexibility in regulating the different compositions and fabricating complex geometry when compared with traditional manufacturing methods. In this paper, polydimethylsiloxane (PDMS) polymer is modified with a thixotropic filler, such as silica and conducting additives, such as carbon nanotubes (CNT) and carbon black (CB) to investigate the viscoelastic ink for the DIW printing. Different amounts of silica in the range from 10-20% are considered. Based on the percolation threshold (1.5% for CNT and 15% for CB), hybrid conducting additives of CB in the amount 1-2wt% are considered to fine-tune the viscosity and rheology of the ink which eventually transform the ink into an electrically conductive while exhibiting shear thinning characteristics. The resulting inks are used to print different cross-sectional geometries and further optimized for further use as piezoresistive sensors. It is found that silica filler in the amount beyond 15% puts a limit in the 3D printer as it takes a significant amount of force to extrude. It is found that the addition of inhibitor greatly decouples the curing process, which indeed assists in printing more complex structures over a few hours. Mechanical and piezoresistive data of some selected inks shown potential as sensor applications in soft robotics and pressure sensing.
Exploring Multifunctional CNT-Epoxy-Glass Fibers Composites
In this work, we explored the enhancement of mechanical properties of microwave absorption composites using multi-walled carbon nanotubes (MWCNTs), epoxy, and glass fiber (GF) reinforcement. We investigated the tensile strength, morphologies, dielectric permittivity, and electromagnetic (EM) wave absorption properties (in a frequency range from 3 to 26 GHz) of the composites. The tensile strength of the composites was enhanced to about 400 MPa, which is comparable to that of commercial Aluminum alloy 6061 (about 300 MPa), whereas, the tensile strength of the pristine MWCNT-epoxy composite is around 15 MPa. The mass density of the MWCNT-Epoxy-GF composites from this work was measured to be around 1.6 g/cm3, while Aluminum alloy 6061 has a mass density ~ 2.7 g/cm3. In addition, the MWCNTs (9 wt%)-epoxy-GF composite presented a strong EM wave absorption in a wide frequency band from 15 to 26 GHz, while Aluminum alloys do not have much EM wave absorption. The high microwave absorption together with the much enhanced mechanical strength of the composites provide a potential as multifunctional composite for some applications.
Thermoresistive Characterization of Monofilament Composites Based on Carbon Nanotube Yarns
The effect of embedding a single carbon nanotube yarn (CNTY) in epoxy resins with different curing temperatures and the effect of a thermal program is addressed by studying the thermoresistive response of CNTY/epoxy resin monofilament composites. Heating-cooling ramps ranging from room temperature (RT~25 °C) to 110 °C are applied to CNTY/epoxy resin composites. CNTY embedded in epoxy resins curing at room temperature (RT) and high temperature (HT, 130 °C) showed a negative dependence of electrical resistance with temperature during the heating-cooling program. The average temperature coefficient of resistance during the heating process found for epoxy resin monofilament composites using HT curing was -7.1x10-4 (±0.16x10-4) °C-1. For epoxy resin monofilament composites cured at RT the temperature coefficient of resistance was -5.26x10-4 (0.10x10-4) °C-1. This small difference may be attributed to an increased density of the polymer chains of the epoxy resin curing at HT, as well as to the level of infiltration of the viscous resin within the external bundles of the CNTY. At the end of the heating process, the fractional change in electrical resistance of the CNTY monofilament composites containing the epoxy resin curing at HT and RT were -8.3 % and -4.7 %, respectively, indicating a higher thermoresistive sensitivity for the epoxy resin curing at HT.
A Study on Effect of Auxeticity on Impact Resistance of Carbon Nanotube Reinforced Composite Laminates
It’s a scientific consensus that carbon nanotube (CNT) with outstanding mechanical and physical performance compared with traditional carbon fiber has great potential on the development of next generation advanced composite materials. Like the way that carbon fibers reinforce the matrix, the alignment of CNTs can also be realized in the matrix, which indicates that the material properties of carbon nanotube reinforced composite (CNTRC) can be customized to achieve large anisotropicity. In this paper, the design and optimization of stacking sequences of CNTRC laminates will be investigated through theoretical and numerical methods to obtain out-of-plane negative Poisson’s ratio (NPR), which is an anomalous material property enabled by the large anisotropicity. The out-of-plane Poisson’s ratios of a CNTRC laminate can be derived based on an extended classical laminate theory (CLT), where the out-ofplane strain and stress are also included. The proposed theoretical method is verified by the comparison with three-dimensional finite element results. Through theoretical analysis of various stacking sequence, we can design and optimize the auxetic properties for CNTRC laminates. Furthermore, the von-Karman type dynamic equations of a laminate beam are also derived in the frame work of Reddy’s shear deformation theory and solved by means of a two-step perturbation technique. To study the low-velocity impact response of auxetic CNTRC laminate beams, the motion equation of the impactor is also taken into account. The contact process is estimated by a modified Hertz model where the effect of out-of-plane Poisson’s ratio is included. The governing equations for both the beam and impactor are numerically calculated by Runge-Kutta method. Finally, the effects of NPR as well as CNT volume fraction on the low-velocity impact response of CNTRC laminate beam are studied and discussed.
Understanding Dynamic Wicking and Curing of Epoxy Resin in Carbon Fiber Tows for 3D Printing Application
In recent years, additive manufacturing (AM), also known as 3D printing, has been developed as one of the major fabrication technologies for advanced composite materials. 3D printing offers a great way of producing composites with costefficiency, scalability, high productivity, and design flexibility. Among the numerous materials used in 3D printing, thermoplastic polymers with discontinuous reinforcement (i.e. powder, nanoparticles, short fibers) have been used successfully thanks to high processability. However, the thermoplastic/discontinuous fiber composites are often limited to extreme circumstances requiring high service temperature or mechanical properties due to the low glass transition temperature of the polymers or discontinuous reinforcement. Thermosetting polymer/continuous carbon fiber composites can be an alternative, but there has been no reported AM technique addressing the process of thermosetting polymers and continuous carbon fibers for direct 3D printing. A fundamental challenge of fabricating such composites is in controlling the irreversible viscosity change of thermosets. If a drastic increase of the polymer viscosity occurs before infusion, it is hard to infiltrate the polymers into the fiber bundle, whereas under-cured polymer cannot sustain a designed pattern during the process. Herein, we analyze dynamic capillary-driven flow generated by thermal gradient along carbon fibers based on the understanding of dynamic wicking and curing of epoxy resin in carbon fibers. It gives a significant breakthrough in overcoming these obstacles by controlling the viscosity and degree of cure of the thermosetting polymers. With numerical analysis, we prove that different liquid absorption capability along the fibers induces dynamic wicking of the resin. And in computational simulation, in-composite heating system (heating along carbon fibers) exhibits the abilities to heat the resin uniformly acting like internal heating and cure the composites with a high degree of curing. And we apply the dynamic capillarydriven flow to the 3D printing for manufacturing thermosetting/continuous carbon fiber composites. The great mechanical properties of the printed composites show that dynamic capillary-driven flow is applicable in fabrication of the composite.
Design of Optimum Aircraft Structure Inspired by Dragonfly Wings
The aim of this study is to establish a novel aircraft design approach combining structural optimized biomimetics. The intention of utilizing biomimetics lies in acquiring an efficient and novel composites airframe structural concept replacing the “black metal structure.â€ A typical macroscopic structural layout on a dragonfly wing, called Voronoi patterns, was applied to the reinforcement structures on a part of an aircraft wing. This design approach significantly contributed to structural weight reduction. The manufacturability of more complex structure is also confirmed by producing prototypes, evaluating quality of created models and identifying an effective sectional shape in In-Situ Consolidation process. Furthermore, this application has advanced to a novel design methodology defining reinforcement layout not as some feature extracted from dragonfly wings but as theoretical and mathematical disposition. This methodology allows composites cover to acquire excellent properties, namely lightness and stiffness combining themselves properly. Bionic structure with Voronoi patterns has potential for weight reduction replacing conventional composites aircraft structures.
Printability of Highly Viscous Composite Slurries via Reactive Extrusion Additive Manufacturing
Additive manufacturing (AM) can enable the fabrication of strong and stiff composite parts with complex geometries and without the need for expensive tools and molds. Current composite AM technologies are slow, energy-intensive, and their resulting parts usually suffer poor inter-laminar properties. This paper investigates the printability of carbon fiber filled thermosetting polymers via a novel AM technique, reactive extrusion AM (REAM). In REAM the resin and curing agent from two separate reservoirs are pumped and mixed directly inside the nozzle and deposited layer-by-layer onto a print-bed. The process utilizes exothermic resin/catalyst systems with fast curing cycles, eliminating the need for post-curing and external energy sources. Chemical cross-linking occurs between the printed layers, resulting in parts with relatively isotropic properties. Viscous properties of the thermosetting polymer are tuned using additives (e.g., fumed silica) to ensure printability while achieving sufficient storage modulus/shear yield strength to hold the shape after printing. Similarly, carbon fibers can be added to tune the viscosity and reinforce the resin. While high carbon fiber loadings are desired for improved mechanical properties, it is difficult to print highly loaded resins due to their excessive viscosity. An integrated experimental/numerical study is performed to examine the printability of a highly viscous 30 wt.% carbon fiber filled resin based on the rheology and pump pressure required for the REAM. Temperature and shear thinning were found to control the process and allow for tuning the rheology of the slurries for REAM printability.
Influence of Stitch Parameters on the Fracture Energy of Stitched Sandwich Composites
The influence of stitch density, linear thread density, and facesheet thickness on the fracture energy of stitched sandwich composites was investigated using a design of experiments approach. Sandwich composites are composed of two rigid facesheets that are bonded to a lightweight core. The separation of facesheets produces a high flexural stiffness that is typically needed for flight-control surfaces in aerospace applications. These structures can develop facesheet-to-core separation under relatively low out-of-plane loads due to their low interfacial properties between the constituents. In this study, sandwich composites were manufactured with through-thethickness reinforcements, or stitching, to act as crack-growth inhibitors. Stitched sandwich composites were manufactured with various treatment combinations of stitch density, linear thread density, and facesheet thickness. Single cantilever beam tests were performed to estimate the mode I fracture energy. A response surface model based on a face-centered design of experiments approach was developed to investigate the selected factors, and their interaction, on the mode I fracture energy. Unique fracture surface morphologies are exhibited during mode I testing that are dependent on stitch processing parameters. The results indicate that stitching is an excellent candidate to inhibit crack growth.
Gecko Inspired Adhesive Patterned Single-Lap Joints
Biology inspires innovation throughout the fields of science and engineering. Inspiration can be found from a dolphin's aerodynamic shape creating low resistance while swimming to a gecko's unique patterned feet, enabling adhesion to almost all surfaces. In industry, lightweight and high strength structures are desired due to their high specific strength performance. To accomplish the desired requirements, nontraditional methods of joining, such as adhesive joints, are often demanded. Adhesive joints play a vital role in today's aerospace applications, as they allow the joining of dissimilar materials to achieve the desired high energy-saving performance. The properties of the adhesive significantly affect the strength of the joint, where hightoughness adhesives can be more beneficial for longer overlap lengths, but less beneficial at high-temperatures; while low-toughness adhesives provide high strength and stiffness over short overlap lengths, but exhibit sudden brittle fracture at lowtemperatures. Hybrid adhesives can combine both high strength and high ductility over a wide range of temperatures and be used to control crack propagation. This work numerically and experimentally studies the effect of gecko-inspired surface patterns on the damage growth and failure mechanisms in a Titanium (Ti) and Carbon Fiber Reinforced Polymer (CFRP) adhesive single-lap joint (SLJ). Finite Element Analysis is performed utilizing Abaqus® paired with Cohesive Zone Modeling to observe the damage growth of the hybrid adhesive based on the material directionality. By implementing directionality to the adhesive properties, the rate at which adhesive failure occurs can be controlled, and the failure mechanisms of the SLJ can be observed. This study applies the concept of biomimicry to evaluate the effect of gecko-inspired hybrid adhesive patterning on the failure mechanisms and strength of SLJs, with the aim to path novel designs for composite adhesive joints.
Water Uptake of Fiber Reinforced Syntactic Foams
Syntactic foams, or structured foams, are frequently used in a wide array of systems that demand the low density of a foam, but higher mechanical properties than can be achieved by traditional foam systems. Such applications include core materials in sandwich composites, low density structures for increased buoyancy in naval structures, and various sporting goods materials. Specifically, in the context of naval structures, stability in a submerged environment is of concern. This study examines the water absorption behavior of fiber reinforced syntactic foams. Samples were manufactured on a pilot scale sheet molding compound (SMC) manufacturing line. Glass bubbles (GB) of various diameters were used in order to elucidate the effect of GB surface area to volume ratio. Tensile testing of hydrothermally aged samples was also conducted to determine the effect on tensile modulus and strength of these composites.
Continuum Damage Modeling of Common Feature Test Component Under Constant Amplitude and Block Fatigue Loading
Progressive damage and failure analysis (PDFA) methods have been proposed as an essential tool for reducing the certification costs of primary and secondary composite structures. Successful application of PDFA methods have been demonstrated on test coupons and relatively small composite components; however, to achieve the goal of cost reduction for certification programs, performance of PDFA methods must be verified at larger length scales than typical aerospace allowable tests. The United States Air Force Research Laboratory (AFRL) has been actively investigating the use of these methods for aerospace durability and damage tolerant applications through the Composite Airframe Life Extension (CALE) program. Prior programs have examined applications such as unnotched tension/compression, open hole tension/compression, bearing, filled hole compression and bearing bypass. The Boeing-led CALE Project 4 seeks to build on the previous efforts and assess the tools for durability and damage tolerance of advanced composite structural features. In particular, this program focused on a Common Feature Test Component (CFTC) representative of key aircraft design considerations and failure modes. CFTC advanced structural features include a composite skin made of unidirectional tape, a fabric hat stiffener, and a mechanically fastened aluminum rib. In this work, the authors present an evaluation of fatigue simulations by CDMat – a PDFA software developed at the University of Texas at Arlington, used for the progressive damage and failure analysis of CFTC under static and cyclic loads. Both constant amplitude and block fatigue loading are considered, and residual strength capability of the element is predicted. This work will present comparison of CFTC simulation predictions with tests that have been conducted at AFRL including blind predictions of critical failure loads and fatigue damage growth for the CFTC specimens.
Modeling Composite Structural Elements Using Discrete Damage Modeling
Discrete Damage Modeling using the Regularized Extended Finite Element Method (Rx-FEM) for matrix cracking is applied to element-level Common Feature Test Components (CFTC) loaded in static and fatigue. The two CFTC considered utilize a hat-stiffened panel structure loaded at the center via an aluminum loading rib. The complexity of these structural elements required extensive use of Layered 8-node brick elements (LC3D8) in non-critical areas as well as utilization of various mesh refinements and approximate boundary conditions to utilize symmetries of the CFTC geometry. Quasi-static simulations resulted in accurate predictions of the critical damage threshold (CDT), which is the first peak load, and overall peak load of the specimen. Accurate predictions of the fatigue response and namely the extent of delamination and stiffness loss were only possible based on calibrated material properties inferred based on sub element testing.
Overview of Composite Airframe Life Extension Program Project 2: Tools for Assessing the Durability and Damage Tolerance of Fastened Composite Joints
In anticipation of performing service life extension programs (SLEP) for modern tactical airframes in the United States Air Force (USAF) with composite materials, AFRL has funded and continuing to fund several projects on developing various tools for durability and damage tolerance (DADT) assessment of composite structures under a multi-year contract called Composite Airframe Life Extension (CALE) program. Under the “Tools for Assessing the Durability and Damage Tolerance of Fastened Composite Jointsâ€ or simply Project 2, Northrop Grumman Corporation (NGC) developed two sets of tools for bolted composite assemblies: 1) parametric modeling methods for rapid durability assessment; and 2) progressive damage analysis (PDA) methods for detailed damage tolerance assessment.. The development, maturation, and assessment of these two types of tools were supported by testing at NGC and AFRL. This paper will briefly describe the project, key test and modeling results, along with recommendations for future activities in further development of methods for DADT assessment of composite structures.
Repeatability of Progressive Damage Methods
The objective of this paper is to explore the finer points of ensuring maximum repeatability in computational results from modern progressive damage, strength, and life prediction methods for laminated composite materials and structures. Separate sections focus on various input properties and computational solution techniques for such tools. Precision of input property test methods, appropriateness of measured input values for particular failure criteria, and proper accounting of physics-based effects such as residual thermal stresses and geometric uncertainties are discussed. Computational choices regarding numerical solvers (explicit, implicit, other), element types, mesh refinement, convergence strategies, etc. are addressed. Recognition is made of the inherent compromises in terms of run-time, physical behavior modeling, and strength/life prediction accuracy. The difference between code verification and prediction validation is also addressed. Finally, the paper summarizes the overall issues and decisions affecting the repeatability of salient strength/life predictions of complex laminated composite structural behavior.
Progressive Damage Mechanisms in Thick 3D Composite Structures Under Static and Fatigue Loading
The objective of this work is to demonstrate validation of predictive capabilities capturing both damage initiation and growth under static and fatigue loading in geometrically complex three-dimensional (3D) composite structures. The composite structure of interest in the present study resembles a 3D blade spar used in rotorcraft applications. Under both static and fatigue testing, it was shown that damage initiated at the same ply-drop location(s) and grew in the axial as well as hoop directions. CompDam – a state-of-the-art Progressive Damage Analysis (PDA) tool was used to achieve successful correlations of metrics including load-displacement responses and damage modes.
Direct Comparison of the Fibre Break Development in Unidirectional Carbon Fibre Composites Under Tension Predicted by Statistical Models and Measured Using 4D In-situ SRCT
In this work longitudinal tensile tests combined with in-situ Synchrotron Radiation Computed Tomography (SRCT) are analysed to capture the accumulation fibre breaks in unidirectional composites bundles of several thousand carbon fibres. A virtual representation of the same microstructure has been simulated using a statistical strength model. The numerical model has shown to capture trends in the fibre break development for a chosen material system. The predicted strength values are validated against macroscopically measured strength for standardised coupons using continuous end tabs. Model inaccuracies observed in regards to the predicted developments of clustered fibre breaks are analysed and an outlook on the further improvement of the strength model is given.
Stochastic Resin Injection Simulation of the High-Pressure Resin Transfer Molding for an Automobile Floor Using Adapted Polynomial Chaos Expansions
Carbon fiber composite materials are being used at an increasing rate in the automotive industry due to their low densities, superior mechanical properties and great design flexibilities. The recently developed High Pressure Resin Transfer Molding (HP-RTM) process enables high resin injection rates and thereby shortens composite molding cycle times to meet the demanding performance and volume production requirements of automotive components. However, variability is endemic in composite materials and their processing. The development of an accurate injection simulation model of the HP-RTM process is critical for robust process design. In this paper, uncertainty quantification (UQ) for the simulation of the resin injection process of a carbon fiber composite is developed to predict the resin flow and thus the outcome of the composite molding. The uncertainty interplay is modeled using polynomial chaos expansions (PCE). Several material and molding process parameters have been considered for stochastic analysis. This multi-variable stochastic manufacturing problem is challenging to solve computationally, and a previously developed basis adaptation scheme has been used to reduce the computational effort. The UQ development has been implemented on the GM high performance computing system, and the coupled UQ toolbox PAM-COMPOSITETM injection simulations have been conducted for an automobile underbody floor. The numerical results show an excellent convergence of the PCE methodology for the chosen injection problem. A hypothesis of predicting the occurrence of potential dry spots indirectly, by monitoring the information about the resin arrival time, the value of pressure increase, and the value of saturation pressure at a location in the proximity of the flow front closure has been proposed and validated.
High-Dimensional Uncertainty Quantification and Global Sensitivity Analysis of a Composite Wind Turbine Blade
In this paper, a stochastic framework for uncertainty quantification and global sensitivity analysis of a composite wind turbine blade is presented. Nowadays, wind turbines blades are made up of composite materials and it represents a complex structure with varying material, chord, and twist distribution along the span of the blade. In fact, manufacturing the wind turbine blades to its exact specifications is a challenging task since various sources of uncertainties are introduced unwantedly during design, manufacturing, transportation, and operation. These uncertainties can have adverse effects on the performance and reliability of wind turbine blades and must be accounted for during the design phase. However, performing uncertainty quantification while considering a large number of random parameters as in wind turbine blades is still a computationally-intensive process since it demands a large number of finite element analysis (FEA) calls. Therefore, a computationally efficient and high accuracy approach called polynomial chaos expansion (PCE) with l1-minimization has been implemented which also allows for global sensitivity analysis to estimate the influence of random parameters on the stochastic responses. The presented approach was tested by considering the randomness in material, geometric, and loading conditions, and the influence of randomness on natural frequencies and failure of an NREL 5MW wind turbine blade was assessed. The results demonstrated that it can provide high-accuracy information regarding the variation of the responses as well as the influential random parameters at an affordable computational cost. Therefore, this study will be influential in understanding the structural behavior and improving the reliability of composite wind turbine blades.
Stochastic Modelling of Additively Manufactured Structures Using a Neural Network for Identification of Random Field Parameters
Heterogeneous materials exhibit considerable spatial variations in properties, impacting structural performance and local stress and strain fields. Recent research has focused on considering material behaviour uncertainties and quantifying the impact of uncertainties on the structural response, requiring the definition of random fields for describing the material variability. In this paper, a stochastic modeling methodology for additively manufactured structures is implemented (forward model). The stochastic parameters are determined from experimental Digital Image Correlation (DIC) images (inverse problem) using a Convolutional Neural Network (CNN), and the CNN is trained using the forward model. Validation of the CNN estimates using previously unseen data shows adequate performance of the network, and consistent predictions are found when estimating the stochastic parameters from experimental results. The proposed methodology allows examination of the stochastic response and uncertainty quantification of additively manufactured structures, while requiring only minor experimental efforts to fully define the random fields. Once the CNN is trained, computational expense for predicting stochastic parameters is minimal.
Understanding Variability in Mechanical Behavior of Adhesively Bonded Sandwich Joints Due to Stochastic Material Damage
A mesoscale progressive damage analysis was applied to the analysis of recently tested adhesively bonded sandwich panels, in which ten nominally identical tension loaded panels failed in three different failure modes, with peak loads ranging from 13.0 –16.1 kips. A parametric finite element model for progressive damage analysis of an adhesively bonded joint (ABJ) considering matrix and fiber damage, and delamination cracks in unidirectional and fabric composite sections of the joint, as well as core crushing, and adhesive fracture was developed. The model was developed using Abaqus/Explicit and utilized continuum damage mechanics material models for intralaminar damage in the facesheet and doubler plies, cohesive elements for delamination and adhesive debonding, and discretized cells considering plasticity to model crushing of the aluminum honeycomb core. Analysis predictions obtained with a pristine model agreed well with the average experimental peak load, strain in the joint, and post-test damage states. Then, various methods to incorporate stochastic material variability into ABJ model were evaluated. These methods included simulation of pre-existing matrix damage, pre-existing adhesive porosity, and uniform modification of adhesive properties. These material and manufacturing defects changed the damage mode predictions in some cases. The analysis of stochastic strength due to the considered defects revealed unintuitive damage mode interaction in the ABJ, explaining some structure-property relationships observed experimentally.
On the Stochastic Characterization and Prediction of Data-Driven Multi-Scale Materials Models for Composites
The work presented is this paper is carried out towards a project on Integrated computational materials engineering for the development of carbon fiber composite for light vehicles. Here, we focus on highlighting selected mathematical and computational aspects pertaining to the stochastic modeling of composites across a hierarchy of length scales. These aspects have been addressed to control the construction of validated structural performance predictors for composites as functions of microscopic parametric descriptors. We show the significance of developing appropriate prior models of fine-scale properties for the characterization process, i.e. their impact on how to devise reliable macroscopic stochastic predictors and how to infer updated microscopic stochastic models. Specifically, we delineate on the process of modeling the moduli of fiber in tension and compression as well as modeling their statistical dependency in view of macroscopic data-driven model updating process. Tensile and flexural physical experiments at the coupon level are employed for this purpose. The characterization of key quantities of interests by means of polynomial chaos expansion has been requisite for developing explicit stochastic coupling across the hierarchy of length-scales as part of the characterization and the verification processes.
Effects of Material Combinations and Curvature on the Failure of Sandwich Structures
In this study, influence of material combinations and curvature on failure of a curved composite sandwich structure has been investigated numerically and both factors were found to be influential. Tsai-Wu and Puck criteria were adopted to evaluate the core and facesheet laminate failure, respectively. For constant curvature, core failure was found to be the first failure event for sandwich structures designed with PVC core while the initial failure mode switched to inter-fiber failure (IFF) in facesheet when the core material was replaced with Balsa. Curvature was also found to be impactful by contributing to high through-thickness normal stresses in the curved region of sandwich structures. A decrease in curvature would lead to the reduction of magnitude in through-thickness stresses, which could eventually result in the switch of failure mode from core failure to IFF for sandwich structures designed with PVC core. Finally, it was found that sandwich structures with more unidirectional fabric content in facesheet are more prone to the initiation of IFF.
High Fidelity Analysis of Dynamic Stresses in Wind Turbine Blades
The preliminary feasibility analysis of using Rocstar Multiphysics for high fidelity modeling and simulation of wind turbines is demonstrated in light of a simple wind turbine problem. To address some of the issues of existing open-source offerings in the multiphysics modeling of wind turbine systems, Rocstar integrates highly featured, well maintained, community-driven flow field and structural solvers. Governing equations are discussed along with the boundary conditions alongside the setup of a simple demonstrative problem. It is shown that by proper choice of materials and modifications to the structure, it is possible to limit the deflections and stress levels in turbine blades. The study is also featured by several analyses of the steady-state and dynamic response of the CFRP and GFRP blades to the wind load under normal and harsh atmospheric conditions.
Wind Turbine blades are constantly exposed to extremely adverse environmental conditions throughout their operation. As a result, they inevitably suffer damage from erosion, exposure as well as more acute incidents such as lightning strikes and extreme wind speeds. Damage must be repaired in the shortest possible time frame to reduce blade downtime. However, current repair processes are highly variable and not optimized for the unique geometry of each repair or environmental conditions. The goal of this study is to optimize the cure cycle to produce a homogeneously cured repair patch in the shortest time and to identify key cure cycle parameters, such as heating rate, cure temperature, hold time, etc., that significantly affect the time and quality of the repair. The effect of each cure cycle parameter on the repair time is investigated individually within a combined experimental-numerical framework. Extensive material characterization of the neat resin is conducted. First, the resin and the curing agent are separately exposed to environmental conditions to measure the moisture absorption capacity. Then, the curing behavior of the resin is characterized using Differential Scanning Calorimetry (DSC) as a function of different environmental conditions. Lastly, Finite element (FE) curing simulations of a single-side scarf repair, are performed within Abaqus through user-written subroutines, to investigate the effect of cure cycle parameters and environmental conditions on the blade repair time. The effect of each parameter is quantified in terms of peak exothermic temperature, spatial distribution of cure and temperature within the repair and the total repair time. Experimental validation in lab conditions is proposed to validate the FE and to gauge the quality of the repair resulting from the optimized cure cycle.