Imaging delaminations in composite laminates via linear-to-nonlinear steady-state wavefields

Abstract

Delaminations in composite laminates, though often barely visible, can compromise the integrity and safety of composite structures. Over the past decade, linear steady-state wavefields (LSWs) have emerged with attractive merits for imaging delaminations. However, LSWs struggle to effectively image small delaminations as their stiffnesses can hardly be affected. In contrast, contact-induced nonlinearity, arising from the breathing behaviour of delamination interfaces, can provide nonlinear delamination features in nonlinear steady-state wavefields (NSWs), offering the potential to provide complete configurations of delaminations. This study focuses on extracting nonlinear delamination features using a nonlinear perturbation to an NSW. The perturbation, which exists exclusively within the delamination region, is virtually treated as a multi-tone harmonic excitation source, serving as an ideal index for imaging delaminations. Particularly, a baseline-free method is utilized to reconstruct the nonlinear perturbation using the NSW data in a statistical manner. The capability of the approach is validated experimentally on a glass fiber reinforced polymer laminated plate with multiple local delaminations, which were induced by Teflon inserts, low-speed impacts, and local heating. A PZT actuator is centrally mounted on the plate to generate a single-tone harmonic excitation, and simultaneously, a scanning laser Doppler vibrometer measures the LSW and NSW of the plate. The experimental results reveal that the NSW can provide additional nonlinear delamination features that are scarcely carried in the LSW. Beyond three large thermal delaminations, several small delaminations induced by the Teflon inserts and impacts can also be graphically characterized, allowing for comprehensive imaging of the multiple delaminations. Copyright