Application of two dimensional Morlet wavelet transform in damage detection for composite laminates

Wavelet transform as a novel signal processing method for singularity detection has been introduced into vibration-based nondestructive testing (NDT) methods. However, for plate-like structures, the mode-shape-based NDT methods depend on the powerful two-dimensional (2D) singularity detection method. The 2D continuous wavelet transform (CWT) based on 2D Morlet wavelet function has high sensitivity to singularity in certain direction, but its anisotropic behavior strongly limits its application in damage detection for plate-like structures. In this paper, a modified 2D Morlet wavelet function is proposed to make the 2D CWT behave like a quasi -isotropic transform and show high sensitivity to damage in all directions. After a series of numerical simula-tions are conducted to investigate the performance of the modified 2D Morlet wavelet function and verify its effectivity in damage detection, a damage identification procedure for composite laminates is experimentally conducted and damage index (DI) is also determined by the modified wavelet coefficient. Results indicate that the 2D CWT based on the modified 2D Morlet wavelet can provide a baseline-free NDT method based on MS, and its effectivity and robustness are experimentally confirmed.