Resonant air coupled inspection of glass fiber reinforced polymer composite plate using single sided access of near and far side of low velocity impact damage

Abstract

Resonance based nonlinear techniques works efficiently in detecting small size damages in the composite structures. Local defect resonance (LDR) is a resonance based nonlinear technique which enables to activate the damage directly. In this study, recently developed LDR based automated methodology is carried out from far side of the impact damage to detect the LDR frequency and mode shape of the impact damage. This methodology is implemented by carrying out experiments on glass fibre reinforced polymer (GFRP) plate using air coupled emission (ACE) technique. This plate contains three artificial delaminations and one low velocity impact damage. Initially, the LDR frequency and mode shape of each damage is accomplished using ACE testing, justified with Laser Doppler Vibrometry (LDV). Later, the Otsu’s algorithm is performed to get the accurate size of the impact damage. Subsequently, a broadband backward sweep is used to detect all delamination including the impact damage. Then, maximum amplitude of each frame has been found from output data of all spatial nodes. Later, the LDR frequency frame ( N L) has been identified in the time domain from the slope and curvature plots followed by Discrete Fourier Transform (DFT) to detect the LDR frequency and mode shape of the damage. So, it has been observed that less than defect size is obtained when inspected from the far side. Further, this method will be extended to find out the depth of individual damage.