A near-field 30–40 GHz millimeter-wave phase imaging method for non-destructive testing and evaluation

Near-field millimeter-wave (NMMW) imaging is a key technique in nondestructive testing and evaluation in critical sectors like aerospace, valued for its deep penetration capability and spatial resolution. However, conventional methods that rely on absolute reflectivity exhibit insufficient sensitivity to subtle depth variations, such as those indicative of bonding defects and internal air voids. To overcome this limitation, this study introduces an advanced imaging method that leverages the phase information of the complex reflectivity. Specifically, the proposed method employs a near-field Synthetic Aperture Radar (SAR) algorithm to reconstruct a complex reflectivity map from the backscattered data, from which the wrapped phase is extracted. Subsequently, a weighted ${\ell }_2$-norm minimization phase unwrapping method is applied to transform the wrapped phase into a denoised, unwrapped phase image, significantly improving imaging performance. A 30–40 GHz prototype system was developed for experimental validation. Tests on materials including metals, polytetrafluoroethylene (PTFE) with prefabricated defects, quartz, and silicon nitride confirmed the method’s effectiveness. Crucially, experiments on PTFE specimens and specimen with bonding defects and cracks demonstrated the enhanced capability of the phase-based approach in detecting depth-related defects. This research enhances the capability of millimeter-wave imaging for the nondestructive testing of defects such as poor bonding.