Non-destructive testing of internal cracks in concrete using low-frequency ultrasonic array frequency wavenumber domain method combined with pulse peak delay factor

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.