Monitoring of impact damages in composite laminates using wavelet transform

Low-velocity impact damage is a major concern in the design of structures made of advanced laminated composites, because such damage is mostly hidden inside and cannot be detected by visual inspection. It is found that the acoustic emission (AE) waves generated by impact loads are undistinguishable from each mode and amount of damage by the conventional analysis methods in time or frequency domain. The wavelet transform (WT) can decompose the AE signals in time and wavelet scale domains, and catch the differences in these waves. It enables to distinguish the damage modes and size. This paper presents the application of the WT to detect the impact damage. As a fundamental approach, the characteristics of the AE signals due to matrix cracks and the evolution of free-edge delamination in [±45 2/0 2/90 2] S Gr/Ep laminates under tensile load were analyzed by the WT. Then the characteristics of impact damages of quasi-isotropic laminates were studied using the WT.