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According to damage detection of strand wire structures covered by anchor clip, an ultrasonic guided wave damage imaging detection method based on dual time reversal was suggested. It utilizes the characteristic of the guided waves traversing the whole tested region and the characteristic of time reversal making the signals gathered in the source at the same time. First, a modulation signal is excited by piezoceramic transducers at one end of the detected region of the strand and the reflection signals and the transmission signals are acquired. Then, a non-defective finite element simulation model corresponding to the detected strand is established. The extracted reflection and transmission damage signals can be imported respectively into the non-defective simulation model after time reversal processing to re-spread. The propagation of the reversal signal in the simulation model causes the model nodes to produce a displacement. These displacements can be extracted separately and calculated by dual time reversal operation, which means multiplying the nodes of the same coordinates at each of the two results. Finally, the result of the focusing operation is accumulated at all times and the damage region can be calculated. The focus results are presented by the point cloud. Experimental results show that the difficulties of traditional time focusing in strand wire structures can be overcome, and the damage region can be imaged preliminarily with a small relative error.

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The damage detection of messenger cable is an important guarantee to ensure the safety of electrified railways. The traditional nondestructive testing methods have difficulty in accurately detecting the damage of messenger cable in cladding zone. In this paper, a novel cross-sparse representation (CSR) method based on dispersion dictionary for ultrasonic guided wave (UGW) is proposed for the outer stranded wire of messenger cable in cladding zone. Firstly, the dispersion curves of UGW in multilayer spiral structure are solved by semi-analytical finite element method, to select an appropriate excitation signal. Secondly, the stationary phase approximation method is used to construct dispersion dictionary according to the dispersion curve which can characterize the modes and propagation distance of UGWs, and the CSR is proposed to separate and identify the damage signal. Thirdly, the simulation models of different damages in the outer stranded wire of messenger cable in the cladding zone are established to analyze the correlation between the damage rate and damage index in the cross-sections. Finally, the damage recognition effect based on CSR is analyzed and compared by experiments. The experimental results show that compared with the sparse representation (SR) method using traditional dictionary, the proposed method can improve the average accuracy of damage signal recognition from about 20% to 99.4% within a wide threshold range.

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