Lamb waves boundary reflections in an aluminium plate for defect detection related to structural health monitoring.

The aim of this investigation is to develop a post-processing technique for Lamb wave SHM enabling the identification of mode reflections related to the plate boundaries, and assess their contribution to damage characterisation. The objectives are to: (1) identify propagation paths followed by the waves reflected from the edges and received by the sensors; (2) enable image reconstruction of plate boundaries; (3) assess how the waveform, and especially the wave packets related to edge reflection, are affected by defects (4) identify damage-induced reflections to assist the characterisation of damage within the entire component. The experimental procedure requires the collection of A-scans in a pitch-catch configuration from a circular array of sixteen piezoelectric disk transducers (acting as either sensor or actuator) bonded to an aluminium plate. A tone burst signal tuned for the fundamental symmetric mode (S0) is transmitted and the resulting signals are processed with MATLAB to obtain the distance travelled by each wave packet. The reflected propagation paths can then be determined based on the distance travelled, the transmitter and sensor location, and the theoretical reflection behaviour. The contribution of the different paths to the waveform is experimentally assessed by introducing coupling defects and observing the changes they induced on the wave packets. Features of the plate edges, and waveform reflections are characterised resulting in a significantly larger inspected region.