High frequency ultrasonics for precursor damage assessments in CFRP composite structures

Accurate detection of early-stage fatigue damage in composite materials is essential for structural integrity assessment yet remains challenging due to microscale damage initiation. While guided wave ultrasonics has been widely employed, its sensitivity at low damage levels is limited. This study presents an alternative approach based on high-frequency P-waves (> 10 MHz) to characterise early-stage damage in Carbon Fibre Reinforced Polymer (CFRP) composites. Using high frequency P-waves along the thickness direction, nonlinear ultrasonic parameters and sideband peak modulation are evaluated in CFRP specimens at ~20% of fatigue life. Specimens are subjected to tensile–tensile fatigue loading four-layer woven CFRP composite, with three damage induction stages separated by 8-hour relaxation periods to simulate fatigue evolution. The results show that nonlinear features in P-wave are sensitive to microstructural changes such as matrix cracking, fibre breakage, and edge delamination. Notably, sideband peak analysis showed higher sensitivity in detecting early-stage damage and outperformed conventional harmonic indicators in capturing subtle changes in material states. Ultrasonic assessments are validated using residual strengths, confirming high-frequency P-waves as reliable indicators of precursor damage in CFRP composites. These findings demonstrate the potential of P-wave–based nonlinear ultrasonics as a viable alternative to guided wave for early-stage damage evaluation in composites.