Damage identification in composite laminates and sandwich structures using ultrasonic guided waves and a 3D laser vibrometer

Abstract

This thesis addresses the feasibility of using ultrasonic guided waves (UGWs) to detect and characterise the barely visible impact damages (BVID) that can develop in thin composite laminates and composite sandwich structures (CSS) by carrying out a fundamental investigation into wave-damage interaction. The interaction of UGWs with BVID in a structure is analysed using full wavefield data obtained by a Laser Doppler Vibrometer (LDV) and by numerical simulations. Multiple signal and image processing techniques are proposed to enhance the features relating to damage. The findings from this analysis are then incorporated into an in-service structural health monitoring (SHM) methodology using a sparse network of piezoelectric transducers. For the laminated composite panels, isolated subsurface delaminations between plies and complex BVID caused by a low velocity impactor are investigated. Both cases show that the first symmetric mode, S0, causes mode conversions when interacting with the defects whilst the first anti-symmetric mode, A0, mainly causes a change in phase and amplitude across the defects. Both cases also show that as the damaged area got more severe, the effects of the damage became more pronounced. The findings are then integrated and validated by a delay & sum algorithm to show the UGWs potential as an in-service SHM methodology. The focus of research then turns to the theoretical fundamentals of UGW propagation through CSS. The underlying mechanics of UGWs in CSS, including the relation between panel thickness and the UGW wavelength as well as the energy transfer through the core are presented. It is noted that three main types of propagation can exist in CSS which are global Lamb waves, leaky Lamb waves and Rayleigh waves. Dispersion curves are obtained for the CSS and polar plots of group velocities show their anisotropic nature. The final part of the thesis focuses on damage detection and localisation in CSS using full wavefield analysis and a sparse network of transducers. Both fundamental modes can localise the BVID in the structure, even with the anisotropic behaviour of the core. Based on these results, this thesis concludes that UGW based SHM shows great promise as an in-service damage detection technique that can detect, localise and, in some cases, characterise impact induced defects in thin composite laminates and CSS.