Observing breakage in sand under triaxial and oedometric loading in 3D

The mechanisms involved in grain crushing/particle breakage are a common research topic in several fields including geomechanics, geoscience and particle technology. This topic is of interest to disciplines including material processing, minerals and mining engineering, geology and geophysics, since changes in the microstructure can lead to significant changes in the macroscopic bulk behaviour of a material. The main goal of this research is to develop a new understanding of the particle breakage mechanisms in a particulate system under specific loading regimes. This is achieved by studying the microscopic changes that an assembly of grains undergoes while being loaded/deformed. The possibility to study and visualise the 3D volume of a specimen during loading is given by x-ray computed tomography. The material under study is a sand (Caicos ooids) which consists of very rounded particles that grow from fragments of corals (Aragonite) with a D50 of approximately 350μm. This material has been subjected to triaxial and oedometric loading, in order to compare the response of the material under different loading regimes. The acquisition of 3D images enables the study of the deformation of the microstructure and especially the evolution of grain breakage at different loading increments. The high resolution of the images enable the authors to clearly observe and perform algorithmic measurements of broken grains in small fragments. The image processing involves the creation of algorithms to track each individual grain, match the fragments to the intact labeled grains, in order to precisely describe the deformation throughout the loading process. The comparison between the oedometric and triaxial loading shows a clear difference in the distribution of damage. In the first case a dispersed pattern of breakage can be observed, indicating that breakage originates predominantly from the boundary conditions (especially the moving boundary – loading platten) and transmission of normal forces between particles, whereas in the second case a very clear strain localisation has been observed, which indicates breakage is mainly due to the deviatoric loading and observed mostly in a narrow shear band.