Wave scattering from discontinuities related to corrosion-like damage in one-dimensional waveguides

Guided waves at high frequency are used in structural health monitoring systems to detect damage. The successful application of the technique, however, requires a deep understanding of wave mode propagation and scattering, as this allows the correct choice of frequency range and wave types to be used to interrogate the structure. Often, this is not the case and systems are developed in an ad hoc way. There is, thus, a greater need to understand the way in which guided waves interact with, and are scattered by, damage. This paper aims to contribute to knowledge in this area. A wave finite element method is applied in a study of wave scattering from symmetric and asymmetric changes of cross-sectional area over a finite length in a waveguide, which are intended to be crude models of corrosion damage. To facilitate analysis of the results, Lamb wave scattering is first investigated in such a system for a point discontinuity involving a change in cross-sectional area. The wave scattering characteristics are evaluated by using the time-averaged kinetic energy of the reflected and transmitted wave fields considering components in the vertical and horizontal directions. This, together with the traditionally used displacement and energy flow coefficients, allows an investigation into the rich wave scattering behaviour due to multi-mode behaviour and wave mode conversion which can complicate the interpretation of the results. In particular, it is shown that care needs to be taken in the interpretation of a sudden increase in the amplitude of a reflected wave from damage. At certain frequencies, this can be due to evanescent waves starting to propagate, rather than an increase in the severity of the damage.