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Development of a Standard Reference Geometry for XCT

TWI Wales has designed and manufactured a standard reference geometry for use with X-ray computed tomography (XCT) by the European Space Agency (ESA). The reference geometry will allow the verification of image quality within the XCT inspection system prior to use. This is of particular interest to industry as currently there are limited standard reference geometries for use within XCT due to the 3-dimensional nature of the acquired data.

A design was developed based on a series of criteria and constraints, informed by a literature review. The programme of work culminated in the manufacture of a standard reference geometry with modular features to allow the verification of image quality using multiple metrics and material types.

Project Background

There is an increasing need for the use of standard reference geometries with XCT as the non-destructive evaluation technique becomes further standardised and utilised within industry. XCT reference geometries will allow the standardised assessment of image quality as has been the case for many years within film and 2D digital radiography.

Designing and manufacturing these reference geometries relies on establishing the method to measure the image quality of a volumetric image. Currently, film and digital radiography rely on image quality indicators (IQIs) which are used in line with current international standards. The 3D nature of XCT does not allow for direct translation of these IQIs for this technique.

 

Work Programme

A literature review was carried out to assess the current state of the art practices for standard reference geometries and IQIs for all radiographic techniques. The review findings were used to design three new geometries for further consideration and development.

Taking into account these findings, and the requirements from ESA, the potential geometries were subjected to a set of weighted evaluation criteria, which resulted in the final design (Figure 1). 

The final design is based around a step cylinder comprised of aluminium, with removable internal cylindrical pins that are raised within the part to represent different penetrable thicknesses. In order to assess the image quality, micrometre scale line features were placed in a circular pattern around the removable pins along with hemispherical patterns to represent porosity. Figure 2 shows an XCT image of the manufactured step cylinder.

This design offers a flexible platform for expansion and adjustment in terms of size, resolution and material. The modular design will allow the investigation of image quality for various penetration pathways of the XCT system as well as for multi-material components.

Figure 1. Final design
Figure 1. Final design
Figure 2. XCT image of the manufactured step cylinder
Figure 2. XCT image of the manufactured step cylinder
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