Nondestructive Testing Using mm-Wave Sparse Imaging Verified for Singly Curved Composite Panels

Nondestructive testing of composite materials is important in aerospace applications, and mm-wave imaging has been increasingly used for this purpose. Imaging is traditionally performed using Fourier methods, with inverse methods being an alternative. This communication presents a mm-wave imaging method with an inverse approach intended for nondestructive testing of singly curved composite panels with sparsely distributed flaws. It builds on previous work which was limited to imaging on planar panels. The move from planar to singly curved panels increases the applicability of the method for aerospace applications. The imaging method is reference-free due to a numerical source separation algorithm and exploits sparsity in reconstruction of scatterers. It is demonstrated using near-field measurements at 60 GHz of an industrially manufactured composite panel with deliberate flaws. Compared to a more traditional Fourier imaging method, our method generates images with higher resolution and higher dynamic range. Flaw detection is also easier using our method as it generates images with less background clutter.