Non-destructive 3D grain imaging in aeroengine materials by diffraction contrast tomography

Traditionally X-ray images rely on attenuation contrast which means that some regions of a radiograph appear light because they attenuate more of the X-rays than others. At present, nearly all lab scanners exploit this contrast mechanism. There are many materials such as carbon fibre composites and soft biological tissue which contain only lowly attenuating materials. In such cases it is difficult to see key features of interest such as defects, cracks and cancerous tissue.

While these soft materials show little attenuation contrast, they do affect the phase of the X-rays through differences in refractive index. Normally these changes in phase cannot be imaged but for highly coherent sources such as synchrotron sources phase contrast is fast becoming a major tool. Laboratory X-ray systems however typically have little coherence and so the application of this method is not straightforward. New methods for revealing phase contrast in lab X-ray imaging will be investigated as part of this project with the aim of developing a prototype phase contrast imaging computed tomography (CT) system, along with the software required to reconstruct the phase contrast images.

This prototype imaging system will then be developed, advanced and tested using a number of key demonstrator materials science problems. The first of these will be the examination of large carbon fibre composite panels. Both manufacturing damage and barely visible impact damage are a major concern for the composite industry especially in aerospace. In particular, unusually large grains are believed to be the route of a number of life-limiting effects, such as dwell fatigue in aeroengine materials. This significantly affects productivity in terms of aeroengine component manufacture and life. This PhD will be the first to apply the phase contrast technique to find, image and follow the degrading effect of these grains and use the mathematics of rare events to accurately life such parts. The application of the phase contrast technique can also be used on other soft solid samples (possible soft tissue samples for example).