Frequency-domain defect imaging in anisotropic media based on coordinate transformation

Ultrasonic testing methods exhibit great potential for nondestructive evaluation in various industrial materials including composite materials. However, the wave-propagation behavior in composite materials becomes complicated due to the combined effects of anisotropy and dispersion, which brings great challenges to defect detection in anisotropic media. In this work, the method for imaging defects in anisotropic materials is proposed by combining phase-shift migration (PSM) and coordinate transformation. As a demonstration, the Lamb wave propagation in highly anisotropic composite plates is analyzed, and described with a two-dimensional equivalent model, which is established by mathematically mapping the anisotropic wave propagation into an equivalent isotropic form. After obtaining the ultrasound field at the scanning positions corresponding to the transformed coordinates, the PSM is applied to image defects in such equivalent isotropic systems. The actual defect position is retrieved according to the geometric relationships. This work demonstrates the feasibility and application potential of the proposed method in the field of defect evaluation in anisotropic composite materials.