Measurement of crack-induced non-propagating Lamb wave modes under varying crack widths

This study aims to measure non-propagating Lamb wave modes (NPLs) for detection of a surface-breaking fatigue crack. Crack-induced NPLs are the attractive feature to identify and localize an invisible crack, because they are not spatially propagated but significantly contribute to local Lamb mode amplitudes at a crack interface. By extracting only the crack-induced NPLs from Lamb wavefields measured at multi-spatial points, the crack is able to be identified and localized without relying on baseline data obtained from the pristine condition of a target structure. First, the existence of the crack-induced NPLs are analytically proved using a modal expansion method. Then, a NPL filter based on a frequency-wavenumber analysis is proposed so that only NPLs are isolated from Lamb wavefields. Subsequently, the theoretical proof and the proposed NPL filter are numerically and experimentally validated through fatigue crack detection tests. In particular, the quantities of NPL generation at the real fatigue crack interface are investigated under changing driving frequencies as well as crack widths. The numerical and experimental results confirm that NPLs remarkably appear at open crack interfaces, while they are rarely induced at closed crack interfaces.