Assessment of out-of-plane ply waviness in carbon-fibre reinforced plastics: Comparing different non-destructive evaluation modalities

Out-of-plane waviness (ply wrinkles) reduces tensile and compressive strength in Carbon Fibre Reinforced Polymers (CFRPs), with maximum out-of-plane ply angle governing failure mechanisms. This study comparatively evaluates three Non-Destructive Evaluation (NDE) techniques: Eddy Current Array Testing (ECAT), Phased Array Ultrasonic Testing (PAUT) and X-ray Digital Tomosynthesis (DT) for detecting and characterising ply wrinkles across three parameters: amplitude, wavelength, and maximum out-of-plane ply angle. Nine unidirectional CFRP coupons containing induced ply wrinkles of controlled amplitudes (0.13–1.31 mm) were inspected, addressing a critical gap in comparative NDE performance for sub-2 mm amplitude defects in thin laminates.

PAUT achieved the highest overall characterisation success rate of 96.3 % (26/27 measurements) and a detection success rate of 88.9 % (8/9 samples). Critically, PAUT achieved 100 % success in characterising maximum out-of-plane ply angle - the parameter governing compressive/tensile failure across all samples, including the lowest amplitude wrinkle (0.13 mm). However, systematic overestimation in wrinkle amplitude characterisation occurred (+55.3 % mean percentage error). ECAT achieved an equivalent 88.9 % detection success and 33.3 % characterisation success, successfully measuring wrinkle wavelength (100 %) but unable to quantify wrinkle amplitude or out-of-plane ply angle from complex impedance data alone, positioning it as a rapid automated screening tool. X-ray DT achieved 88.9 % detection and characterisation success, with moderate overestimation in wrinkle amplitude characterisation (+24.8 %). However, complete detection and characterisation failure occurred on the lowest amplitude ply wrinkle.

A critical finding establishes that reliable characterisation requires ply wrinkle amplitudes ≥0.32 mm across all techniques, with implications for the wrinkle parameter hierarchy in manufacturing quality control.