Scattering of the Symmetrical Edge-guided Wave by a Small Edge Crack in an Isotropic Plate

Abstract

Reliable detection and sizing of small fatigue cracks remains a significant challenge for structural integrity management, particularly for cracks at hidden or hard-to-inspect locations. There is significant interest in using Lamb wave propagation for structural health monitoring (SHM) in such cases due to their low attenuation and rapid wide-area coverage. However, unlike bulk waves, Lamb waves are more complicated and require further research, both experimentally and computationally, in order to fully exploit their potential for SHM. This paper presents a computational study of the interaction between the edge-guided wave associated with the fundamental symmetrical mode S0 and a small through-thickness edge crack in an isotropic plate. The incident edgeguided wave is generated by a force applied on the edge and whose time dependence is a 5-cycle Hann-windowed toneburst of centre frequency below the cut-off for the first order Lamb wave modes (A1 and SH1). 2D Fast Fourier Transformation (2D FFT) is performed and was used to produce the dispersion curves to identify the scattered Lamb wave mode. The scattering pattern and dependence of scattering amplitude on crack size are reported. It is shown that the scattered field due to a small crack (i.e. with the crack length, a, is significantly smaller than the wavelength of the incident wave, λ) can be considered to be equivalent to a point source consisting of a particular combination of force doublets, and that the amplitude of the scattered field increases like some power of the crack length a. It is also noted that there is no geometric decay of the edge wave with propagation distance, making them particularly attractive in the SHM context. This study is regarded as a prerequisite for a subsequent attempt to investigate the inverse problem of determining crack size from measurement of the scattered wave field. Contribution of the findings to non-destructive crack detection and characterisation in SHM are also briefly discussed. doi: 10.12783/SHM2015/245