Detection Method for Cavity Defects in Ballastless Track Structures of High-Speed Railways Based on Air-Coupled Ultrasonic Lamb Waves

This study proposes a method for the rapid detection and location of cavity defects in ballastless track structures of high-speed railways in service. First, the propagation of air-coupled ultrasonic Lamb waves in the ballastless track structure is studied. Theoretical calculation results show that the ultrasonic Lamb wave group velocity of the A2 mode in the track plate is 4000 m/s. Then, the excitation and reception methods of the air-coupled ultrasound are studied. Theoretical and experimental results show that the A2 mode Lamb wave can be generated by the 3.8° oblique incidence of the ballastless track structure. Finally, an experimental system for air-coupled ultrasonic testing is constructed. A pair of air-coupled ultrasonic probes is used to provide excitation and reception Lamb wave signals at an inclined angle of 3.8°, 20 mm away from the surface of the track plate, and 40 mm/step along the scanning direction. Experimental data indicate that interaction between the Lamb wave and cavity conforms to the energy leakage principle, and the amplitude of the Lamb wave increases with the increase in cavity of the scanning path. The “position-amplitude” curve is drawn from the collected experimental data. Based on the quantitative relationship between the convex interval of the curve and the size of the cavity that can be calculated to obtain the cavity size, the detection error value is ±5 mm. Theoretical and experimental results show that noncontact rapid detection of cavities can be realized by using the air-coupled ultrasonic Lamb waves.