In situ Advanced X-ray Computed Tomography for geomaterials
Thanos Papazoglou
When thinking of geomaterials, we understand that conventional "point-wise" or "bulk-average" measurement techniques are not representative. In conventional material testing, the specimen's response is characterized only globally and presented as an average material response. Thus, these measurements are rigorously valid only for homogenous samples that undergo a perfectly uniform deformation.
Geomaterials exhibit a variety of complex microstructures that evolve with time and pressure. Furthermore, neither the geomaterials are homogeneous nor are boundary conditions ideally applied. These arguments have made the full-field and Non-Destructive Testing techniques (NDT) well-established tools for geomaterials.
Among many full-field measurement techniques, X-ray CT can be considered as the reference NDT technique for geomaterials. The 3D tomographic reconstructed images provide unprecedented insight into the nature and complexity of bio-, hydro-, thermo-, chemo-, and mechanical processes happening within a rock mass. X-ray CT is a powerful tool for imaging the surface as well as the internal structure of the geomaterials with a spatial resolution, from 1mm down to a few tens of nanometres. Besides
the laboratory scanners, Synchrotron-based systems can be used. Synchrotron radiation microtomography has emerged as the most powerful NDT technique that provides more advanced imaging techniques such as phase contrast tomography, allowing very subtle structural changes in the sample to be detected. The high flux and parallel beam produced by synchrotron facilities enable both ultra-fast image acquisition and high spatial resolution, at an energy that can be easily adjusted to the sample nature and experiment needs.
The systematic use of X-ray CT combined with advanced image analysis and Digital Image Correlation (DIC) provides valuable 3D information on localization patterning even when deformation is isochoric. When X-ray CT images of a rock sample are acquired before and after, or even during loading, digital image correlation can be used to obtain the displacement and deformation fields.
