Ultrasonic Waveform Inversion and Nonlinear Synthetic Focusing Imaging in Multilayered Complex Structures

Multilayer complex structures are widely used in the energy and power industries. However, due to the combined effects of multiple media layers and complex curved surfaces, using phased array ultrasonic inspection to check their internal structures and defects is still extremely challenging. This paper proposed a two-stage array ultrasonic method for the inspection of internal structures. In the first stage, a low-frequency full waveform inversion was used to characterize the complicated internal structure, overcoming the challenge of a priori velocity estimation while improving computational efficiency by 75% compared to full-spectrum full waveform inversion. In the second stage, a nonlinear synthetic focusing imaging method was utilized to achieve high-resolution imaging of internal defects. To further reduce the computation time for beam path estimation, an Eikonal equation-based method was introduced. The proposed method improves computational efficiency by approximately 96.85% and 93.93% compared to the traditional binary search and Fermat’s principle-based shortest path algorithms, respectively. Experimental results demonstrated that the proposed method can effectively detect internal defects within multilayer complex structures. Compared with the conventional array ultrasonic full focusing method, the global contrast index (CG) value increased by 2.87 times, while the array performance indicator (API) value decreased by 88.73%.