Experimental investigation of inter-particle contact evolution of sheared granular materials using X-ray micro-tomography

The inter-particle contact evolution of two sheared granular materials, i.e., a spherical glass bead (GB) specimen and an angular Leighton Buzzard Sand (LBS) specimen, is investigated non-destructively using X-ray micro-tomography. A miniature triaxial apparatus is developed for the testing with in-situ scanning. Full-field X-ray CT images of the two specimens are obtained at different shearing stages. A series of image processing and analysis techniques in combination with a particle-tracking approach is developed to detect the inter-particle contacts and to determine the contact gain, the contact loss, and the contact movement during each shear increment. It is found that the average coordination number (CN) experiences a strong change in the pre-peak shearing stage, and tends to reach a steady value after the peak. As the shear progresses, the average CN of the particles with different sizes follows the same trend as the overall average CN. Additionally, as the shear progresses, the branch vectors of the specimens, which, prior to shearing, are nearly isotropically distributed for the rounded GB and concentrated along the horizontal direction for the angular LBS, are found to show a directional preference towards the loading direction. The contact gain and the contact loss, which contribute to this directional preference, and the contact movement, which leads to the attenuation of the directional preference, are shown to be the two competing factors determining the evolution of the fabric anisotropy of granular materials. The higher degree of fabric anisotropy in the shear bands is shown to be mainly attributed to the higher percentages of contact gain and contact loss when compared to that of the entire samples.