Low-frequency ultrasonic array is commonly used to detect interlayer voids located in high-speed railway ballastless track, which is a typical multilayer concrete bonded structure. The difficulty of detection lies in the fact that the total focusing method (TFM) based on a single fixed sound velocity model cannot adapt to the acoustic propagation characteristics of multilayer structures, which is prone to generating artifacts. In addition, the long duration of low-frequency ultrasonic pulses is prone to causing significant deviations in defect localization. To address these issues, a theoretical model of the layered bonded structure is proposed. The acoustic wave propagation path and travel time calculation are clarified after combining the Fermat’s principle and Snell’s law, and the shortest path ray tracing (SPRT) is proposed, which achieves visual imaging of interlayer voids; The pulse peak delay (PPD) is applied to correct the travel time of low-frequency ultrasonic waves, and the shortest path ray tracing combined with pulse peak delay (PSPRT) is proposed, which significantly improves the localization accuracy of defects. Finally, by integrating the amplitude and phase information of scattered signals, the shortest path ray tracing based on pulse peak delay and sign coherence factor (PPSPRT) is constructed, which significantly enhances the SNR. The test results show that, compared with the conventional TFM, the proposed PPSPRT achieves average SNR improvements of 6.62 dB in numerical simulations and 14.30 dB in field tests, and reduces the average depth localization error of interlayer voids to merely 23.49% and 10.38% of that of TFM under corresponding test conditions, respectively. PPSPRT can provide important guidance for accurate imaging of interlayer voids.
Fan et al. (Thu,) studied this question.