Accurate measurement and testing of parts with complex internal features can be fraught with difficulties. Often, the only way to achieve accurate checking is to sacrifice a part to destructive testing in order to ensure correct internal tolerances are being met.
However, even this cannot ensure complete control over the quality of finished parts. For example, the make-up of parts produced using SLS (selective laser sintering) processes can vary from one component to the next, so a means of non-destructively examining components internally can greatly reduce wastage. One such method that is seeing increasing use is CT (computed tomography) scanning, which allows for accurate and detailed analysis of both internal geometries and material density.
Most people will be aware of the term CT scanning thanks to its widespread use in medicine, but its use in an industrial context is less well known. Put simply, a CT scanner uses an X-ray source to produce tomographic images, or ‘slices’, of a component, which are then processed to create a 3D image of the component’s internals. The first generation of CT scanners took nine days to produce a single slice image; now though, even highly complex objects can be scanned in less than an hour thanks to advances in computer processing power.
A CT scanner for industrial use will feature an X-ray source (an open or closed X-ray tube) that produces a conical beam of electrons that penetrates the object to be analysed. The electrons are detected and interpreted by a 2D sensor as a ‘digital radiograph’ image. The object being examined is positioned on a precision rotary stage, and an image is acquired each time the stage is rotated. Depending on the resolution required, each step is usually 0.1-1º, producing between 360 and 3600 individual images. The scan usually covers a rotation of 360º, but for specific applications the scan can be a limited angle.
Once a component has been scanned, the resulting image can be used to study different parameters. For example, if a scan was taken of a cast component then variations in material density throughout the part can be examined. With software, problems such as inclusions in the material, cracks or voids left during the casting process can be highlighted. Whereas previously the only way to check for these would have been through destructive testing, CT scanning allows for any component to be checked and then used. Leading on from this application, CT scanning also opens up a route to effective reverse engineering of components. The most common form of reverse engineering is laser scanning, however, this has limitations in that it is hard to measure internal geometries. With CT scanning, this is not an issue and with the correct analysis software, highly accurate 3D models can be created from scans, which can then be transferred into regular CAD packages.
One particularly interesting application for CT scanning in the motorsport environment is in checking composite structures. The strength and stiffness of structural carbon fibre parts is determined by both the orientation of the carbon fibres and the fibre-to-resin ratio. When designing such parts, engineers will use FEA (finite element analysis) to determine the optimum weave arrangement; however, it is impossible to check how closely the final part conforms to the design parameters without using destructive test methods. Using CT scanning, the exact internal structure of the part can be checked and thus its real-world mechanical performance more accurately predicted.
These are only a few of the potential uses for this interesting technology, but it has the potential to provide engineers with a highly accurate method of checking a plethora of previously hard-to-examine parts. The only downside is cost – CT scanners are very expensive, so having such a facility in-house is not feasible for most operations. (which is why most CT inspection is performed at 3rd party CT inspection labs as opposed to in house inspection). However, with an ever-increasing number of parts made using 3D printing, and a subsequent need to closely examine internal forms is likely to be an increase in third-party suppliers providing CT services, making it easier to outsource parts for checking.
Written by Lawrence Butcher