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.

The total focusing method (TFM) is an ultrasonic phased array imaging algorithm used in ultrasonic nondestructive testing (NDT) that processes large amounts of data from full matrix capture (FMC). This limits its application in some industrial fields with real-time requirements. To solve this problem, a sparse array optimization method is applied to FMC-TFM that can reduce time consumption and improve imaging efficiency. However, conventional intelligent optimization methods such as genetic algorithms use binary encoding, which require intensive computation and are easily trapped in local optima. This paper proposes a discrete slime mold algorithm (DSMA) in which the slime mold position is coded in real numbers instead of binary. In the optimization process, a mapping model between the slime mold and ultrasonic array is established. A fitness function with a narrow main lobe and low side lobe is constructed to obtain the sparse array position with the best performance. In experiments, the proposed method reduces the imaging time by more than 50% compared with conventional TFM, without affecting imaging quality. Compared with a genetic algorithm (GA) and binary particle swarm optimization (BPSO), the proposed method improves API and SNR performance.

The current Lamb wave topology imaging methods are limited to the ideal model of through-hole defects, ignoring the influence of mode coupling. However, blind hole defects, such as structural plate corrosion and cracks, are the most likely defects in practical applications, and Lamb waves are bound to appear mode-coupled at these defects. The use of the full matrix data calculations of the direct and concomitant sound fields is time-consuming and limits the industrial application of the ultrasonic Lamb wave time-domain topological energy (TDTE) imaging method. A TDTE imaging method of a sparse ultrasonic Lamb wave is proposed in this paper to solve the aforementioned two problems. The sparse array is designed based on a genetic algorithm, then the direct sound field and the accompanying sound field are calculated using the sparse array data. The mode conversion of the Lamb wave is considered when calculating the accompanying sound field, and then the accompanying sound field is reversed in time so that the direct sound field and the accompanying sound field are adaptively focused at the defect. The influence of Lamb wave mode conversion and dispersion on the imaging results is overcome. Finally, the TDTE values of the direct sound field and the accompanying sound field are used as the imaging pixel values to improve the imaging accuracy. The experimental results show that for asymmetric blind hole defects, the TDTE imaging method is more accurate than the total focus (TFM) imaging method; the sparse array designed by the genetic algorithm has good sidelobe characteristics and can effectively reduce sidelobe noise assuming the array aperture is unchanged. The TDTE imaging results based on the sparse array data show that the imaging efficiency is greatly improved compared with the full array when the receiving array is four elements under the condition of ensuring the imaging quality.