Pure flexural guided wave excitation in pipes using cross-exciting circumferential phased arrays

Flexural guided waves that preserve the circumferential focusing characteristics provide insights into the structural integrity and thus have great application potential. An improved transducer called cross-exciting circumferential phased array is proposed for generating pure flexural mode in pipes. It is divided into two cross-exciting sub-arrays, with opposing excitation angles with respect to the pipe axis, forming a standing wave in the circumference and thereby a clear circumferential focus. The excitation response of the proposed transducer is theoretically analyzed with the normal mode expansion method. Finite element simulations of flexural mode excitation with the proposed transducer and a conventional circumferential array are further compared. The simulation results agree well with the theoretical predictions. Pure flexural modes of circumferential order of one are successfully generated. The excited signals are comprehensively analyzed. Furthermore, purity improvements are investigated with respect to the excitation frequency, element number, element width, and axial length. The excitation frequency should be appropriately chosen to suppress the unwanted modes. The element number should be sufficiently large to prevent the excitation of high circumferential order modes. The element width has negligible effect on the target flexural mode purity, whereas the element axial length significantly should be carefully chosen.