Ultrasonic guided wave focusing in waveguides with constant irregular cross-sections.

As essential components of a high-speed railway system, switch rails can be easily damaged by sophisticated operating conditions. Therefore, precise online detection for switch rails is necessary. Methods based on ultrasonic guided waves are ideal candidates for structural integrity of the switch rails, which are natural waveguides with irregular cross-sections. However, energy decentralization in the wave propagation severely restricts detectability. Phased array systems have been developed and implemented to steer and focus acoustic energy in waveguides of ordinary cross-sections such as pipes and plates. This paper proposes a method for ultrasonic guided wave focusing in waveguides with constant irregular cross-sections. We analyzed the characteristics of the guided waves generated by partial loadings based on a semi-analytical finite element method (SAFEM). An algorithm was developed for calculating the amplitude weights and time delays required to modulate excitation signals. Two coefficients were defined to evaluate the wave focusing results, namely the half area coefficient (HAC) and half energy coefficient (HEC). Numerical simulations to verify the proposed method were carried out for a switch rail base with a constant irregular cross-section. The results demonstrate that the guided acoustic beam has been effectively steered to focus at the pre-determined locations with enhanced acoustic wave energy. Furthermore, the influence of various factors on guided wave focusing was studied. Excitation signals of low center frequencies with narrow bandwidths are recommended for ideal focusing results.