Numerical Investigation of Ultrasonic Phased Array Reverse Time Migration Technique Considering Spatial Wave Characteristics

This paper presents a systematic numerical research on ultrasonic phased array reverse time migration technique for damage evaluation in bulk materials. In this study, a thick aluminum bulk is used as the target structure to be tested, and an ultrasonic phased array composed of piezoelectric elements and damping blocks is prosed as the transducers for generating and receiving elastic waves. Firstly, a Finite Element Model (FEM) of a pair of transducers is established to study the wave generation and reception performance. In particular, the suppression effect of different backing material parameters (damping ratio, thickness, implementation details) on the piezo-elements to absorb excessive resonant vibrations is investigated, in order to send out and receive spatially squeezed mechanical pulses into the target medium. Then, a full-scale FEM is established with the complete probe set and typical structural damage types to understand the wave propagation and its interaction with damage. Both longitudinal (L) and shear (S) waves are studied, while they interact with a hole and cracks with different orientations with respect to the incident wave direction. Finally, the reverse time migration algorithm is further developed by considering the spatial wave characteristics. The amplitude variation along the propagation distance is taken into account to form a time/space-gain compensation function to improve the damage imaging quality and sensitivity, especially for far field damage sites. At the same time, the imaging algorithm is tested for the single L-wave, the single S-wave, and the fused LS-wave scenarios. It was found that the combination of L-mode and S-mode can significantly improve the damage imaging results. This numerical investigation may lay a solid foundation for the development of ultrasonic phased array technique for non-destructive evaluation (NDE) of bulky materials. This paper ends with a summary, concluding remarks, and suggestions for future work.