Thinned array ultrasonic imaging of debonding defects in discontinuous impedance bonded structures

Ballastless track is a typical bonding structure with discontinuous impedance. Under the influence of heavy load, deteriorating environment and other situations, the circumstance of debonding occurs frequently and seriously affects the safety of train operation. Because the ultrasonic detection based on the constant sound velocity ignores the difference between the acoustic impedance of the mediums, focusing the acoustic beam at the defect becomes difficult and results in large errors. Although, the position and shape of debonding defects can be obtained by using the full matrix acquisition method based on the linear periodic array combined with thefull matrix data imagining this method heavily depends on dealing with a large amount of data and presents the imaging inefficiently. In this connection, a high-precision array ultrasound fast imaging method is proposed. Based on the propagation path and time of sound wave in the medium obtained by ray tracing method, a theoretical model of sound velocity of discontinuous impedance bonding structure is established when the difference of sound velocity among dielectric layers is taken into account. High degree of freedom sparse array based on real number coding as well as SAFT(synthetic aperture focusing technology) imaging method are used to improve detection efficiency. The results of the experiments carried out on ballast less track structure show that: The ray tracing method can accurately calculate the propagation path and propagation time of ultrasonic wave and improve the detection accuracy. The optimized thinned array pattern has narrowed the main lobe width and low side lobe gain, which improves the detection efficiency and sound field directivity. The efficiency of thinned array SAFT imaging method is improved by 30.9% when the imaging error of debonding defects is within ± 5%, which provides theoretical support for the detection of such debonding defects.