Development of Offset X-ray Computed Tomography Inspection
By G Liaptsis and A Clarke
X-ray Computed Tomography (XCT) is a widely used tool within many industrial sectors for the Non-Destructive Testing (NDT) of various materials/parts, including developmental components as well as pre-production and production items. XCT can provide information to engineers such as the detail of the structure of composites, manufacturing tolerances and deviations (geometrical and volumetric) of complex additive manufactured components, and the presence of defects within finished items destined for assembly. One of the greatest limitations of an XCT system is the size of component it can handle, which is restricted by the field of view of the high cost digital X-ray detector.
This report describes work carried out to develop TWI’s capability and know-how, through development of solutions to allow the Computed Tomography (CT) inspection of large component sizes on existing XCT systems. The term large component refers to any object with width that, even with the lowest magnification, does not fit within the field of view of the digital detector. The solution investigated was offset CT, which is used in academia but, currently, only to a limited extent within industry.
- TWI has developed sufficient understanding of offset CT to implement and deploy the inspection method.
- The use of the tailing technique, with various implementations of data completion, allows larger components to be inspected, extending the capability of existing equipment.
- The sigmoid function provides a promising method of tailing, enabling greater control over the simulated projections and the subsequent reconstructed offset CT data.
- There are limitations to the immediate application of this approach due to the need to use a proprietary Feldkamp, Davis, and Kress (FDK) algorithm. This is being addressed by development of an in-house FDK algorithm.
Offset CT acquisition setup.
Cross-section of a large polymer composite joint using data from an offset CT scan:
a) With no processing of X-ray images, showing artefacts due to the offset;
b) After use of the tailing technique, showing removal of artefacts due to the offset.