Quantification of grain breakage during creep based on X-ray microtomography

Delayed compression is among the leading causes of long-term deterioration in granular systems, especially when it is mediated by the action of pore fluids. This time-dependent process is often classified as ‘creep’, a term conveying time-dependence without specifying the causes of deformation. This paper presents a methodology based on X-ray synchrotron microtomography to track delayed microstructural changes in compacted sand. Experiments on materials characterized by different grain size and shape have been designed to measure macroscopic variables such as strain rate, as well as to visualize the topological and morphological alterations of the constituting particles. The results reveal that non-negligible inelastic processes such as grain breakage manifest during the first stages of loading, as well as during the ensuing constant-stress delayed compaction. A substantial role of the grain morphology was detected in both stages. Specifically, while samples made of angular grains displayed early breakage due to the exacerbated fragility of the particles, specimens made of rounded grains did not develop a markedly polydisperse structure prior to creep, which led to comparably more intense delayed fracturing. Furthermore, samples consisting of round grains were also found to exhibit more intense shape alterations, with morphological indicators that tended to converge over time towards those of initially angular grains. These results suggest that characterization and simulation of creep in granular media need to encompass a variety of microscopic processes controlled by grain-scale properties, thus requiring multi-scale testing and modelling techniques