Precise internal geometric characterization of multilayer structures using ultrasonic array imaging technology

The growing use of multilayer components in manufacturing demands precise inspection methods. Ultrasonic phased array imaging with full matrix capture (FMC) offers effective structural characterization. However, its application to multilayer media is fundamentally constrained by the accuracy of sound velocity measurements. This paper introduces an ultrasonic phased array full matrix imaging method based on frequency domain full waveform inversion, which achieves precise reconstruction of the internal geometries characterization of multilayer structures by only utilizing the sound velocity of the outermost medium. Through an iterative optimization process, the initial sound velocity is progressively refined to minimize the discrepancy between simulated and experimental FMC datasets, thereby ensuring the simulated sound velocity closely approximates the actual conditions. This innovative method achieves robust reconstruction of internal geometries by eliminating the traditional requirement for accurate sound velocity measurements. The influence of unknown wavelets on internal geometries reconstruction was suppressed effectively by applying Green's function in the objective function. Furthermore, a novel pseudo-Hessian matrix incorporating Green's function correction is derived to enhance illumination compensation in deeper regions of the component. The experiments and simulations of different designs of three-layer medium complex structures were conducted, in which the proposed method provided substantially improved visualization of internal features. The structural similarity (SSIM) was increased 1.6 times higher than the conventional techniques, and the root mean square error (RMSE) of geometric dimensional measurements was reduced by more than 30 %.