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

传统上，X射线图像依赖于衰减对比度，这意味着X光片的某些区域看起来很亮，因为它们比其他区域衰减更多的X射线。目前，几乎所有的实验室扫描仪都利用了这种对比机制。有许多材料，如碳纤维复合材料和软生物组织，只含有低衰减材料。在这种情况下，很难看到关键的特征，如缺陷、裂缝和癌变组织。虽然这些软材料表现出很小的衰减对比度，但它们确实通过折射率的差异影响了X射线的相位。通常情况下，这些相位变化不能成像，但对于高度相干的源，如同步加速器源，相位对比度正在迅速成为一种主要工具。然而，实验室X射线系统通常几乎没有相干性，因此这种方法的应用并不简单。作为该项目的一部分，将研究实验室X射线成像中显示相衬的新方法，目的是开发相衬成像计算机层析成像(CT)原型系统以及重建相衬图像所需的软件。然后，该原型成像系统将利用一系列关键的演示材料科学问题进行开发、改进和测试。其中第一项将是对大型碳纤维复合板的审查。制造损伤和几乎看不见的冲击损伤都是复合材料行业，特别是航空航天行业的主要担忧。特别是，异常大的颗粒被认为是许多寿命限制效应的途径，例如航空发动机材料的驻留疲劳。这大大影响了航空发动机部件制造和寿命方面的生产率。这位博士将是第一个应用相位对比技术来发现、成像和跟踪这些颗粒的退化效应，并使用罕见事件的数学来准确地使用这些部件的博士。相衬技术的应用也可用于其他软固体样品(例如，可能的软组织样品)。

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).