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Adhesively bonded metals are increasingly used in many industries. Inspecting these parts remains challenging for modern non-destructive testing techniques. Laser ultrasound (LU) has shown great potential in high-resolution imaging of carbon-reinforced composites. For metals, excitation of longitudinal waves is inefficient without surface ablation. However, shear waves can be efficiently generated in the thermo-elastic regime and used to image defects in metallic structures. Here we present a compact LU system consisting of a high repetition rate diode-pumped laser to excite shear waves and noncontact detection with a highly sensitive fiber optic Sagnac interferometer to inspect adhesively bonded aluminum plates. Multiphysics finite difference simulations are performed to optimize the measurement configuration. Damage detection is performed for a structure consisting of three aluminum plates bonded with an epoxy film. Defects are simulated by a thin Teflon film. It is shown that the proposed technique can efficiently localize defects in both adhesion layers.

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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.