Delamination detection in composite laminates using high-frequency P- and S-waves - Part I: Theory and analysis

A structural health monitoring (SHM) technique that exploits piezoelectric-induced ultrasonic waves propagating across the thickness of composite laminates is proposed to detect the presence of inter-layer delaminations. The damage-induced lag in the times of arrival of primary (P) and secondary (S) elastic waves is exploited as the key mechanism to enhance the detection capabilities. The theoretical aspects of the design of the proposed SHM procedure for thin-walled multi-layered structures are presented first. Two- and three-dimensional numerical analyses of the wave propagation across the thickness of composite laminates with a single delamination are carried out. A correlation between the delamination position and the percent variations of the time of flight (ToF) of P- and S-waves is found. Strong modulation of the power spectral density (PSD) of the output signals is obtained for delaminations close to the surface onto which the actuator–sensor pair is mounted. While for the latter no additional information about their position in the laminate stacking sequence is obtained, for deeper delaminations the proposed procedure delivers very accurate estimates of the damage location. The experimental validation of the described delamination detection technique is carried out in the second part of this study (Pasquali et al., in press [1]).