Study of generation and propagation of ultrasonic trailing pulses in rail web

• The generation and propagation of trailing pulses in rail web are analyzed and validated. • Ultrasonic morphology evolves with changes in the thickness-to-wavelength ratio. • The trailing pulses may be one of reasons for false alarms or missed detections of rail flaw detection vehicle. The morphological characters of ultrasound influence the pulse-echo inspection of steel rails significantly. Specifically, the steel rail is a complex waveguide, with the rail head, rail web, and rail base are quite different in their dimensions. As the thickness to wavelength ratio changes, the ultrasonic morphology evolves between bulk waves, trailing pulses, and guided waves. Among them, the trailing pulses are a transition regime between bulk waves and guided waves, manifested as a series of pulses which are nearly non-dispersive and equally-distributed in time domain. From the perspective of geometric acoustics and Rayleigh-Lamb function, the mechanism for the generation of trailing pulses in the rail web is analyzed, the time difference between adjacent pulses is solved. The finite element method is then applied to simulate the reflection and refraction of ultrasonic waves at the structural boundaries as they propagating from the rail head to the rail web, demonstrating the generation process of trailing pulses. Experimental results from rail surface and web cross-section validate the theoretical model, showing less than 2% error in pulse time intervals.