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. • Implementation of a robotically deployed Eddy Current Array Testing (ECAT) system to automate the inspection of the Carbon-Fibre Reinforced Plastic (CFRP) laminates affected by out-of-plane ply wrinkles. • Implementation and investigation of X-ray Digital Tomosynthesis (DT) as an emerging technology for rapid inspection of out-of-plane ply wrinkles in CFRP laminates. • A comprehensive tri-modal comparative evaluation of ECAT, Phased Array Ultrasonic Testing (PAUT), and X-ray DT for detecting and characterising out-of-plane ply wrinkles with amplitudes below 2 mm in thin CFRP laminates based on ply wrinkle parameter hierarchy (Maximum out-of-plane ply angle, amplitude and wavelength).