Characterization of microstructure and internal displacement field of sand using X-ray computed tomography

This study presents a systematic method to examine the microstructure characteristics and internal displacement field of sands using X-ray computed tomography (CT) images. The 3-D images of spherical glass beads, Silica sand, and Ottawa sand are characterized using advanced image processing techniques.; An interactive computer program is developed to study porosity variation with increasing radius of a spherical volume from the 3-D images of these materials. The porosity variation of Silica sand and Ottawa sand shows three characteristic regions: an initial fluctuation region due to microscopic variations, a constant plateau region, and a region with a monotonic increase/decrease due to heterogeneity. The homogenous medium of glass beads did not show the last region. The results show that for the spherical glass beads the representative elementary volume radius is about 2 to 3 times the average diameter. The radius for Silica sand composed mainly of elongated particles is between 5 to 11 times of d50 and for Ottawa sand composed mainly of subrounded particles is between 9 to 16 times of d50. These values appear to justify the use of 10 to 20 diameters of sand grains adopted in some past studies.; A novel triaxial system is designed to facilitate the characterization of the 3-D images of soil microstructure and its evolution nondestructively in real time using X-ray computed tomography while it is subjected to shearing. The system was designed to have a total weight of 500 N, capable to increase the cell pressure up to 400 kPa, and applying up to 10 kN axial load. Moreover, the system is designed to be able to do temperature controlled (-10 to 65°C) triaxial and uniaxial tests on many different materials, including soil, asphalt concrete, wood, small metal specimens, and composites. The system is fully microprocessor controlled using a workstation outside of the protection cabinet of X-ray CT. Load can be kept constant, or applied either in strain control or stress control.; A computer code is developed to determine the internal displacement field in sands by comparing two successive X-ray CT images. The method is an extension of the template matching technique used in image processing for 3-D situations. The program is verified by applying a known displacement or rotation to the reference image. An interactive computer program is developed to find the changes in local porosity distribution within the sand specimen as it is subjected to shearing. The ability to quantify the internal displacement and local porosity would contribute in the characterization of strain localization and shear band development.