Guopeng Fan

The total focus method based on a low-frequency ultrasonic array is difficult to achieve the accurate detection of cracks in non-destructive testing of concrete structures due to the complex propagation characteristics of ultrasonic waves. In this study, a pulse peak delay factor is applied to correct the propagation time of low-frequency ultrasound waves. On this basis, the total focus method based on the pulse peak delay factor (PPDF-TFM) is proposed to achieve the accurate depth of cracks. The frequency-wavenumber domain method based on the pulse peak delay factor (PPDF-FWDM) is proposed to improve the transverse resolution of cracks. In addition, the removal of surface waves from received signals can effectively obtain scattered waves of cracks and significantly suppress artifacts in ultrasonic images. Numerical simulations and experimental verifications are implemented to evaluate the detection performances of cracks in terms of the imaging efficiency, signal-to-noise ratio (SNR), and transverse resolution. Test results show that the imaging efficiency, SNR, and transverse resolution of the PPDF-FWDM are significantly improved compared to that of the PPDF-TFM. Further study shows that the PPDF-FWDM not only can quickly and accurately characterize horizontal and inclined cracks with different lengths in concrete structures but also is suitable for detecting interface cracks existing in subway tunnel structures and shows a broad potential for engineering applications.

Resilient wheel is a multi-layer composite structure with discontinuous impedance, the internal structure is prone to damage due to repeated interaction. It is a difficult point for damage detection and imaging of the resilient wheel. This paper proposes a phased array ultrasonic imaging method based on FDMAS-TFM, and high-resolution imaging for defects in the different positions of the resilient wheel is investigated. This method introduces a pairwise multiplication between the delay and summation on the ultrasonic signals, which utilizes the spatial coherence of signals, then the second harmonic component is filtered out for ultrasonic imaging. Finite element simulation and experimental results indicate that the imaging performance of FDMAS-TFM is better than that of the total focusing method (TFM), mainly embodied in higher imaging resolution and stronger noise rejection. For imaging defect inside the wheel rim, the FDMAS-TFM increases signal-to-noise ratio by up to 21.55dB compared with the TFM. For imaging defect at the joint between wheel rim and rubber block, the FDMAS-TFM reduces array performance index by at least 36% compared with the TFM. The results show practical application capability of the proposed method in the resilient wheel.