Evolution of particle morphology and mode of fracture during the oedometric compression of sand

Abstract

This work uses synchrotron X-ray tomography to quantify the role of the particle shape on the interaction between collective comminution and individual grain fracture. Two sands (subrounded Ottawa and subangular Q-ROK#2) were compressed beyond their comminution pressure and imaged at the microscale. An algorithm was developed to track the sequence of breakage events stemming from compression, as well as the evolving morphology of the resulting fragments. The results show that the degree of grain shape evolution depends on the initial morphology of the particles, with subrounded particles exhibiting the most severe shape alterations. However, it was found that at high compressive stress all morphological indices approach similar values. Such a tendency is achieved when crushing involves primarily the finer grain fractions – that is, when cushioning hinders the rupture of larger particles. A quantitative assessment of the mode of particle failure also revealed that in both sands comminution (i.e. the pervasive fragmentation of individual particles) was not achieved abruptly, but was rather attained by way of successive stages of splitting and chipping. These findings suggest that in crushable granular solids the initial grain shape plays a key role during the first stages of compression leading to yielding, but its influence tends to vanish at higher pressures, when cushioning mitigates the impact of initial morphological differences and hinders further major breakage.