Multi-layer imaging method for void defects in ballastless track using forward ray tracing with SAFT

•SAFT with forward ray tracing is proposed to locate void defects in ballastless track.•This method is validated with finite-element analysis and lab experiment.•The method outperforms RR-SAFT and RMS-SAFT.•The research promotes more efficient and precise ultrasonic imaging of ballastless track. Rapid and precise detection of void disease in ballastless track is fundamental to the repair and maintenance of high-speed rail. Under the difference in material attributes of ballastless track in each layer and the resultant refraction, the calculation of sonic wave travel time can be complex. To this end, ultrasonic imaging is developed for the precise and rapid imaging of void diseases in ballastless track with multi-layer concrete structure. Combining forward ray tracing and synthetic aperture focusing technique (RF-SAFT), the ultrasonic imaging mechanism is developed based on the forward mapping of the refracted ray trajectory to establish the relationship between the refracted ray and multi-layer boundary, so as to identify the void locations. This method is calibrated and validated with finite-element analysis and experiment, which is demonstrated to contribute to efficient high-resolution photography of void diseases in multi-layer concrete ballastless track, with the calculation time reduced by 70% in comparison to the conventional backward ray tracing with SAFT, and higher precision than that by root mean square SAFT. This research can promote responsive and precise detection of void disease in ballastless track to support effective repair and maintenance of high-speed rail.