Characterization of Ply-Waviness Manufacturing Defect in Composite Aerospace Laminates by Ultrasonic Scattering Analysis

Abstract

The use of composite materials in the aerospace industry has considerably increased in the last decades. Despite their superior mechanical performance and tailorable nature, they are hindered by their expensive manufacturing processes and complex damage evolution behavior during their lifetime. Among the manufacturing defects that particularly characterize automated methods and/or complex parts, ply waviness in Carbon Fiber Reinforced Polymer (CFRP) could lead to strength reduction and complex behavior, and needs further attention, both in its detection and understanding of damage evolution. Ultrasonic c-scanning techniques are well suited for composite inspection: they are currently used in the aerospace industry for non-destructive evaluation of newly manufactured parts, in- service inspections and maintenance, and can be tuned to detect ply waviness. In this work, CFRP laminate beams specimens with induced out-of-plane ply waviness have been inspected in a water submerged C-scan system, using ultrasonic pulse-echo technique. Data were collected for different specimen parameters and system settings, including ply waviness severity, ultrasonic transducer frequency, and C-scan resolution. A customized code was then developed to process, visualize, and analyze the waveforms collected from the tests. Front-wall, back-wall, and internal wave- packets were identified using estimated calculations of the wave velocity in composite laminates. Then, different waveform features such as time-of-flight and amplitude were extracted to visualize and analyze the ply waviness and its characteristics. It was found that internal and back-wall wave packets experienced a decrease in amplitude over the ply waviness region of the specimen. Furthermore, regions to the left and right of the ply waviness experienced noticeable differences in time-of-flight and amplitude for various wave packets. Additional features and signal processing techniques are employed to consistently characterize ply waviness defects and study their evolution under fatigue and other loads by means of ultrasonic scattering analysis.