Enhanced total focus method for imaging heterogeneous metal bonded structures

The homogeneous media-based total focus method (TFM) has been applied to the nondestructive testing of various industrial components in recent years. However, this technique is inaccurate and inefficient for multilayer medium. To address this problem, this paper proposes an improved multilayer structural ultrasound imaging algorithm based on the Fermat principle. The algorithm linearly interpolates a series of virtual refraction points to the geometrical positions of the transducer elements and the imaging pixel points, determines the delay by calculating the minimum propagation time in all the virtual paths, and applies the calculated delay law to reconstruct the structural image. This method avoids the traditional iterative solution process of delay calculation based on Snell's law, and has good imaging efficiency. The imaging results show that the proposed method can image single or multiple defects in heterogeneous metal bonded structures well.