Localization of Delaminations in Composite Laminates by Nonlinear Interface Stresses

Abstract

While often barely visible, delaminations in composite laminates can critically degrade structural performance and safety. However, localizing small delaminations that minimally alter local stiffnesses remains challenging. On the other hand, contactinduced nonlinearity arises when a delamination periodically closes and opens like breathing, resulting in time-varying normal and shear stresses across the delamination interfaces. This study defines these stresses as nonlinear interface stresses (NISs), emphasizing their application to the localization of delaminations in composite laminates. The NISs exclusively appear within “breathing” delaminations, acting as multi-tone harmonic excitation sources that produce higher harmonics. Additionally, the NISs can serve as ideal indicators for localizing delaminations due to their inherently localized nature. The NISs are reconstructed from nonlinear steady-state wavefields (NSWs) of composite laminates at higher harmonics. The approach based on NISs is experimentally validated on a glass fiber-reinforced polymer laminated plate containing multiple local delaminations fabricated using Teflon sheets, low-speed impacts, and local heating. A piezoelectric actuator, mounted at the center of the plate, generates single-tone harmonic excitations, while a scanning laser Doppler vibrometer measures the NSW of the plate. The experimental results reveal that not only large thermal delaminations but also small delaminations caused by Teflon inserts and low-speed impacts can be spatially characterized with locations and sizes.