Phase Shift Migration Approach Using Full Matrix Capture Optimization for Multi-Layer Ultrasonic Imaging

Abstract Ultrasonic phased array testing is a widely used nondestructive testing method seen in regular inspection of pressure vessels and pressure pipelines. However, some complex structures of multiple layers give rise to impracticality of existing imaging techniques on accurately evaluating the size and shape of defects within inner layer. In this study, a method to measure sound velocity of multi-layer structure is first established based on the full matrix capture (FMC) data, thereby automatically estimating the sound velocity of the coupling layer and ultimately optimizing the phased array focal law for the unknown multi-layered structure. The FMC-based phase shift migration (PSM) imaging method for multi-layer structures is then explored. This method processes the FMC data in the frequency-wavenumber domain and utilizes the wave motion equation layer by layer in the depth direction to realize the imaging of the target layer. In order to resolve online inspection of multi-layered structure (e.g. oil-immersion detection of tank floor), the experiments are conducted on a test specimen covered with sludge. Our finding is that the FMC-based PSM imaging method can realize the high-resolution imaging of the side drilled hole with 1mm diameter under the unknown multi-layer coupling condition. The results suggest that this method could deliver great potential to accurately locate and quantitatively evaluate small buried defects within the inner layer of complexly coupled structure and self-calibrate the fundamental imaging parameter in undiscovered coupling state.