The X-ray view

Article of 27 September 2019 

Non-destructive testing of components

A complicated bone fracture in the leg, a dental implant or discomfort in the spine – in medicine, computed tomography (CT) is frequently used to create cross-sectional images of the human body. This facilitates diagnostics and treatment. CT is also used in industry. It can non-destructively show the interior of components, which benefits both quality assurance and the reconstruction of components.

A well-known sight in doctors’ surgeries and hospitals: the CT scanner. However, the device is not only used to analyse bones and organs; it also plays an important role in industry by providing detailed pictures of the internal structures of components – without dismantling or destroying them.

Measuring, analysing and reconstructing

A main area of application is quality assurance and measurement technology. For example, CT scanners can be used to determine the casting quality of aluminium castings, to check porosity and to calculate the geometric dimensions of components. In the case of injection-moulded plastic components, an additional assessment is made to determine whether there is any warping. Entire component groups can also be examined – for example, to assess whether the spring is correctly installed or whether the seal is working.

Another area is reverse engineering, whereby components are reconstructed for which no CAD model is available. CT generates so-called STL data of the components. The STL format describes the surface of the analysed parts in the form of triangles. These can be used with geometric rules in engineering design to generate new CAD models.

The STL format represents the surface of 3D bodies using triangles. The STL files can be used to create CAD models of components for which none yet exist.
The STL format represents the surface of 3D bodies using triangles. The STL files can be used to create CAD models of components for which none yet exist.

The full view with one scan

The CT scanner takes a large number of 2D X-rays. The three-dimensional volume is then calculated from the data on the computer. At the same time, disturbing artefacts (falsifications) can be removed. A volume generated with this technique is clearer in comparison to the superimposed X-ray image and can be displayed layer by layer.

The X-ray source and detector are fixed in the CT scanner, and only the object under examination is rotated 360 degrees. One of the sticking points is the retainer: it must not interfere with the X-ray beam; at the same time, however, it must secure the component very firmly. Even if the component only moves by micrometres, the measurement will be falsified.

The analysed components have very different sizes – ranging from a few millimetres to several metres. The retainer must be precisely adjusted so that nothing slips.
The analysed components have very different sizes – ranging from a few millimetres to several metres. The retainer must be precisely adjusted so that nothing slips.

Another challenge is that the components often comprise very different materials. A multi-material mix, such as a brass component with a rubber seal, requires an additional software tool to recalculate the volume and minimise artefacts in the image.

CT in use at Festo

At Festo, CT – among other things – is used in the new-product development process. Before new components or changes go into series production, CT analyses are carried out to examine whether the components fulfil the specified properties and quality requirements – for example, whether the casting quality is correct or whether an optimisation must be carried out.

Another field of application is the analysis of defective components. If, for example, leakage occurs, a CT scanner is used to search for the leak – perhaps the cast cover is porous or the rubber seal has been incorrectly positioned. The device is also used for customer complaints. The biggest advantage here is that the components can be analysed non-destructively without the entire product having to be time-consumingly and irrevocably dismantled.