Fracture surface morphology of brittle geomaterials influenced by loading rate and grain size

The fracture surface roughness of grain-based geomaterials is one of the significant factors reflecting the changes in certain physical and mechanical properties. In this paper, the transition of fracture surface roughness of brittle geomaterials with loading rate was investigated theoretically and experimentally. The notched semi-circular bending (NSCB) method was used to clarify the morphology transition mechanism integrating both the macro and micro-fracturing aspects of grain-based structures. Two measurement techniques, high-speed digital image correlation and a crack propagation gauge, were applied to acquire real-time crack locations and crack propagating velocities of NSCB samples under various dynamic loads. Macroscopic failure data were verified via our modified rate-dependent theoretical model for the prediction of crack deflection and penetration. Then the typical parameter, joint roughness coefficient (JRC), was used to characterise the micro-features of each fracture surface at different loading rates. This parameter is quantified in terms of the fractal dimension through our image processing method. In light of the comparisons of three scenarios under different degrees of dynamic loading, the JRC values linearly decrease with loading rate, which is attributed to the predominant failure pattern of microscopic grains changing from deflection to penetration. In addition, micro-grains in larger size on fracture surface roughness are more sensitive to the increase of loading rate, e.g. in rock, resulting in a greater decrease of fracture surface roughness with respect to that in cement with small micro-grains. However, if the crack propagation velocity exceeds the critical value that contributing to the micro-cracks branching, the declined fracture surface roughness will increase monotonically with the loading rate. The results help to predict the failure pattern of two-phase structures in both quasi-static and dynamic conditions as well allowing quantitative analysis of fracture surface morphology under dynamic load.