X-ray microtomography and digital volume correlation for internal deformation and strain analysis

Abstract: A material that is exposed to mechanical load or experience a variation in its immediate environment (temperature, pressure, humidity etc.) will to some extent be affected by these new conditions, which is reflected through structural movements in the material. In order to measure engineering properties related to these structural changes, such as for example deformation and strain, we need to gain information about them that are precise and reliable. There exist many different methods for such measurements, which in most cases are based on the pure surface response due to the deforming mechanism. As long as the material structure is reasonably homogeneous the surface information may be enough but as the complexity of the material structure increase it gets more important to obtain information from the inside of the material. Here, a method for full 3D imaging and quantitative analysis of internal deformation and strain in inhomogeneous materials is presented. 3D structural information from the deforming material is obtained through use of x-ray microtomography. The deformation of the structure is analysed with a 3D pattern recognition technique called digital volume correlation, which is a 3D extension of digital image correlation. A thorough theoretical description of both image formation through x-ray microtomography as well as 2D and 3D structural deformation analysis is given. Complimentary, more practical aspects of the different x-ray imaging systems used in the research are described together with the different methods used for image quality assurance. Four different applications are presented. The first is an example of how rapid processes such as internal granular flow can be imaged and analysed with this kind of methods. The temporal resolution needed to resolve the process yields a sacrifice of spatial information and the analysis is carried out in 2D with digital image correlation. Secondly, the deformation and strain in 3D micro-scale wood structure exposed to three-point-bending is measured by use of synchrotron x-ray microtomography and digital volume correlation. Thirdly, the 3D structural swelling in wood microstructure due to water exposure is analysed using the same methods. Finally, the motion and induced strain in a granular material due to compaction is measured in 3D. The results show good agreement with corresponding 2D measurements, carried out for comparison. The experiments show that the method successfully can be used for analysis of various kinds of deformations and materials and that the results are trustworthy.