On X-ray back-scattering to detect hidden cracks in multi-layer structures

A method that uses X-ray scanning to detect subsurface cracks is described. The method deals with samples typical of aircraft structures for which access is limited to one surface and a crack is hidden within the first substrate layer beneath the surface layer. A single-scatter model for the total response due to all X-rays that scatter once in one of the substrate layers and then pass through the top surface of the crack region and escape the target is developed. In this model, the crack is treated as a region of lower average density, and hence interaction coefficient, than the unaffected bulk material. The EGS4 Monte Carlo code also is used to estimate multiple-scattering response assuming the crack region is a thin void. These two models provide consistent results which indicate that properly designed X-ray back-scatter scanners can identify the presence of hidden cracks that extend through the substrate layer and are greater than about 1 cm in length.