Numerical Investigations of Ultrasonic Reverse Time Migration for Complex Cracks Near the Surface

Metal materials will produce fatigue cracks under long-term cyclic loading. It is difficult to fully image the topography of defects with the current mainstream ultrasound imaging technology. The Reverse time migration (RTM), which has extensive application in geophysical exploration, can image complex geological structures. This research introduces the RTM into the field of ultrasonic non-destructive testing. Through numerical simulation, it is concluded that the RTM based on no absorbing boundary can make the defect contour clearer, but the RTM based on the absorbing boundary can make the image contrast metric higher. In order to obtain the RTM results with high contrast metric and clear defect contours, the solution given in this study is to increase the migration aperture. Then this research discusses the influence of different cross-correlation imaging condition on the imaging quality. And compared with the prevailing total focusing methods (TFM), it shows the superiority of the RTM to the bottom opening crack (BOC) imaging. Finally, the RTM was used to complete the imaging of the curved and bifurcated cracks with complex structures. And after reducing the number of ultrasonic transducers, there are still good imaging results, which can reduce the waste of resources and energy.