On the delay-and-sum processing of shear wave signals from a scanning thermo-elastic laser ultrasound system

Scanning laser ultrasound (LU) systems capture sets of signals from different laser positions that may be combined using delay-and-sum (DAS) processing. The most common model used to calculate DAS delays for point-source systems assumes that ultrasound propagates in circular wavefronts. Vertically polarised shear wave (SV) signals from point-like, thermo-elastic LU sources are expected to violate this assumption due to phase rotation effects and the impact of SV head waves. This work explores the consequences of LU SV-SV wavefront non-circularity on DAS processing. Model SV-SV signal sets from point- and plane-reflectors are shown to interfere destructively during DAS when the circular wavefront model is applied, with a loss of 40% of the available signal predicted for an example plane reflector imaging scenario. Signal sets from experiments reproducing the modelled scenarios validate these predictions, with 51% signal loss observed in the corresponding plane reflector scenario. Phase compensation factors derived from the analytical models are applied to the experimental signals but are not found to improve phase coherence, indicating disparities between the models and experiments. An alternative delay model considering a shuttlecock-shaped pre-reflection SV wavefront is shown to yield signals up to 1.8 times those obtained using the circular wavefront model.