Fatigue Performance of Highly Aligned Short Fiber IM7/LM-PAEK Thermoplastic Composites

Abstract

(R=0.1, 3Hz). In comparison, this significant level of applied max strain (0.9 %) with no micro-cracking is ~50% higher than the initiation strain level for a typical carbon/epoxy having a much lower intralaminar fracture toughness (initiation strain is 0.6% and exhibits crack saturation (24 cracks/cm). The high degree of fatigue damage resistance observed for the 3mm IM7/LM-PAEK TuFF is attributed to the significant level of interface adhesion between the fiber and the resin (Interfacial Shear Strength of 120 MPa) and high matrix ductility (>40% strain to failure) and high intralaminar fracture toughness of the 90˚ layers.Tailorable universal Feedstock for Forming (TuFF) is a highly aligned short fiber composite material (fiber aspect ratio of 600) that can achieve a high fiber volume fraction (50-60%). Previous studies on unidirectional (UD) 3mm IM7/polyetherimide (PEI) have demonstrated quasi-static and tension-tension fatigue properties comparable to continuous fiber composites (57% fiber volume fraction). In this work, UD 3mm IM7/Low-Melt Polyaryletherketone (LM-PAEK) TuFF panels (Vf = 54%) were tested in the fiber direction using a tensile fatigue methodology to generate an S/N curve (R = 0.1, 3 Hz) with periodic checks for modulus loss over a minimum of 1 million cycles. The results indicate 3mm IM7/LM-PAEK is superior to other discontinuous fiber composites and equivalent to other continuous fiber composites in the literature. The comparison is based on the slope of normalized strength versus cycles on a semi-log plot with LM-PAEK exhibiting the lowest slope/strength reduction. No reduction in axial fiber tension modulus was measured. This study further looks at the tension-tension fatigue performance of [0/90/0] IM7/LM-PAEK TuFF cross-ply laminate with a thick ((240 gsm) 90˚ ply subjected to a maximum tensile strain of 0.9% (R = 0.1, 3 Hz). Typically, cross-ply composite layups exhibit transverse cracking in the transverse plies (90˚ layer) that extend across the entire thickness. Models and experimental studies show that the initiation and saturation of transverse cracking during static loading depend on several factors including mechanical loading, residual stress, and ply thickness. The effect of fatigue loading has been shown to reduce the initiation strain. In general, initiation strain for transverse cracks decreases with increases in residual stress and ply thickness (the typical range of initiation strain is 0.1 to 0.6% tensile strain). The key material property at the continuum length scale governing transverse cracking is the intralaminar fracture toughness of the 90˚ layers. In the present study, the thickest 90˚ layers (240 gsm) in the cross-ply 3mm IM7/LM-PAEK TuFF laminates exhibited no transverse micro-cracking up to ~1.5 M fatigue cycles with a 0.9% maximum strain