School of Urban Rail Transportation, Shanghai University of Engineering Science, Shanghai 201620, China.
School of Communication and Information Engineering, Shanghai University, Shanghai 200444, China.
Sensors (Basel). 2019 Oct 28;19(21):4677. doi: 10.3390/s19214677.
High-precision ultrasound imaging of void defects is critical for the performance and safety assessment of ballastless track structures. The sound propagation velocity of each layer in the ballastless track structure is quite different. However, the traditional concrete Synthetic Aperture Focusing Technique (SAFT) ultrasound imaging method is based on the assumption that the concrete has a single constant shear wave velocity. Thus, it is not a suitable method for the ultrasonic imaging of multilayer structures. In this paper, a Multilayer SAFT high-precision ultrasound imaging method is proposed. It is based on the ray-tracing technique and uses the Fermat principle to find the refraction point that minimizes the delay of the acoustic wave propagation path at the interface of the discrete layers. Then, the acoustic wave propagation path is segmented by the position of the refraction point, and the propagation delay of the ultrasonic wave is obtained segment by segment. Thus, the propagation delay of the ultrasonic wave is obtained one by one, so that the propagation delay of the ultrasonic wave in the multilayer structure can be accurately obtained. Finally, the focused image is obtained according to the SAFT imaging algorithm. The finite element simulation and experimental results show that the Multilayer SAFT imaging method can accurately track the propagation path of the ultrasonic wave in ballastless track structures, as well as accurately calculate the propagation delay of the ultrasonic wave and the lengths of void defects. The high accuracy of the Multilayer SAFT imaging represents a significant improvement compared to traditional SAFT imaging.
高精度的无砟轨道结构空洞缺陷超声成像对于其性能和安全评估至关重要。无砟轨道结构各层的声传播速度差异较大。然而，传统的混凝土合成孔径聚焦技术（SAFT）超声成像方法基于混凝土具有单一恒定剪切波速的假设，因此不适合多层结构的超声成像。本文提出了一种多层 SAFT 高精度超声成像方法。该方法基于射线追踪技术，利用费马原理找到使离散层界面处声波传播路径延迟最小的折射点。然后，根据折射点的位置对声波传播路径进行分段，并逐段获取超声波的传播延迟。因此，逐个获取超声波的传播延迟，从而可以准确获得多层结构中超声波的传播延迟。最后，根据 SAFT 成像算法得到聚焦图像。有限元模拟和实验结果表明，多层 SAFT 成像方法可以准确跟踪无砟轨道结构中超声波的传播路径，准确计算超声波的传播延迟和空洞缺陷的长度。与传统的 SAFT 成像相比，多层 SAFT 成像的高精度有了显著提高。
