Ultrasound Imaging of Bottom Opening Crack Based on Frequency-Domain Reverse Time Migration

In 2-dimension (2D), a frequency-domain reverse time migration (FRTM) based on the optimal 9-point finite difference method (FDM) is introduced into the field of ultrasonic non-destructive testing (NDT). This study solves the challenge that conventional ultrasound imaging methods are difficult to image defects completely. In numerical investigations, the use of multiple reflection waves and the necessity of bottom identification are explained. In actual inspection, the method of bottom identification is given, and then the bottom opening crack (BOC) of different lengths in the aluminum block can be completely imaged. Furthermore, the conventional total focusing method (TFM) cannot image the sides of the BOCs using the same data as FRTM. Therefore, the quantitative analysis of BOC based on the complete imaging results of FRTM is more convincing. On this basis, the optimization methods for the number of grid points placed at each wavelength and the frequency calculation range are also given. On the premise of ensuring high image quality, the imaging computation time is reduced to 1/18 of the original, which improves the computing efficiency of the FRTM.