Application of radiographic criteria to Computed Tomography inspection in non-destructive testing

Non-destructive testing of industrial parts using X-ray computed tomography is hampered by the absence of standards defining acceptance criteria for defects generated by the various manufacturing processes available, such as casting or additive manufacturing. To define acceptance criteria for a part inspected by tomography, it is currently necessary to use the standards used for X-ray inspection. A study has been launched at Cetim to test different tomographic image processing methods for applying the criteria defined in radiographic standards. The aim of this study is not to compare both inspection techniques, but to define defect sizing methods using voxel data processing software, and then to assess their relevance to the radiographic criteria applied. Our examination of existing standards for foundry, forging and welding has led us to consider two main types of criteria: acceptance threshold criteria, for which the dimensions of the defect are compared with those accepted by the standard, and visual comparison criteria, which involve comparing an area of the image under examination with a reference image of the same dimensions containing the same type of defect as the one whose harmfulness we are seeking to assess. For each of the two types of criteria, different sizing and comparison methods were tested on a scan of a casting and a scan of a weld. Their relevance was then assessed in terms of level of similarity with a radiographic examination, ease of implementation and level of performance in terms of dimensioning. It appears that not all criteria are systematically applicable in tomography and that they do not offer the possibility of exploiting the 3rd dimension offered by tomography. Finally, recommendations were formulated, as well as prospects for further development, which we feel are essential if we are to take greater advantage of the specific features of X-ray tomography and avoid further penalizing this type of inspection, whose acquisition times are long and already quite costly.