Single mode Lamb wave excitation at high frequency thickness products using a conventional linear array transducer

Lamb wave excitation at high frequency-thickness products offers a potential solution for high-resolution guided wave testing. The method is attractive for corrosion mapping and crack imaging, in particular in locations with limited access. However, multiple modes may propagate, complicating signal interpretation, which is undesirable. In this work, a systematic approach is presented, in an effort to determine the influence of the key parameters related to single higher order Lamb wave mode excitation with a conventional linear array transducer. Specifically, a linear time delay law is used to enhance a targeted mode, while the array's length, pitch and apodisation profile remain to be optimally selected. First, an analytical solution is derived based on modal analysis. This provides a natural decomposition of the amplitudes of a guided wave mode to the product of the response of a single element and the excitation spectrum, which is related to properties of the array. Then, a key observation is made, associating the spectrum to the directivity function for bulk wave phased array steering. This allows the application of well established phased array analysis tools to guided wave phased array excitation. In light of this fact, minimisation of the spectrum’s bandwidth, elimination of the grating lobes and derivation of an apodisation profile are performed, to enhance the purity of the targeted mode. Finally, experiments conducted on an aluminium plate verify the above theoretical results. The Full Matrix is acquired, and all signals are reconstructed synthetically.