Detection of defect in ballastless track based on impact echo method combined with improved SAFT algorithm

Abstract

Impact echo method is a superior method for structural health monitoring in concrete, but it has the disadvantages of low accuracy in detecting multi-layer structures. To overcome this limitation, an improved synthetic aperture focusing technique (SAFT) algorithm was proposed and combined with impact echo method for the detection of ballastless track. The improved SAFT algorithm makes full use of time domain echo signals containing multiple conversion elastic waves to image ballastless track defect. Simulation analysis based on finite difference time-domain (FDTD) method and a laboratory experiment were conducted to verify the effectiveness of improved SAFT algorithm combined with impact echo method for the identification of ballastless track defect. The results show that the decrease of sensor spacing and contact time in impact echo method contributes to the visual identification of defects by the improved SAFT algorithm, the visualization effect of defects can be improved by adjusting the minimum threshold level of the color scale of reconstructed image. Except for defect near the boundary, impact echo method combined with the improved SAFT algorithm has good applicability for the detection of ballastless track defects with random spatial location and number distribution.