Near-field wave enhancement and ‘quasi-surface’ longitudinal waves in a segmented thick-walled hollow cylindrical waveguide

Thick-walled structures with varying geometry are challenging for guided wave inspection due to the multimodal behaviour and the complex scattering of the wave modes. This article investigates the influence of the geometrical transitions on the propagation of the elastic waves in thick-walled cylindrical structures and proposes a structural evaluation technique based on the identified wave phenomena. In particular, a near-field wave enhancement effect caused by the crack-wave interaction and by the thickness changes in a waveguide is explored. Additionally, formation and propagation of the so-called longitudinal ‘quasi-surface’ waves are investigated, as they are found to be a main contributor to the observed wave enhancement phenomenon. The proposed new damage identification technique utilising the enhancement effect is validated numerically and experimentally on a beam and a hollow cylindrical structure.