Experimental test and numerical simulation of the effect of brittleness on the microfracturing of sandstone

The micro-cracks are one of the main causes of rock materials heterogeneity and significantly affect the rock strength and deformation behavior. A detailed study of the micro-cracking process is difficult experimentally due to the complexity of the texture in rock materials. In this study, the flat-jointed model (FJM) was used as a numerical approach based on discrete element method (DEM) in two-dimensional particle flow code (PFC2D) to the quantitative analysis of the effect of brittleness on the micro-cracking behavior of different types of sandstone samples with different brittleness degrees. For this purpose, initially, cylindrical and disc-shaped specimens were made to produce numerical models. Then, the micro-parameters were calibrated based on the macro-parameters of the intact rock specimens obtained from the compression (UCS) and tensile (BTS) tests in laboratory conditions. The stress-strain curves, peak strength, and axial strain values, elasticity modulus, and failure modes as features of numerical simulation are in well agreement with laboratory results. The results showed that the crack initiation, propagation, and coalescence process as sequences of crack evolution are significantly related to the degree of brittleness. The index of the obtained crack growth rate (CGR), for each of the stages of crack development, was increased with brittleness. The period's length of the linear and nonlinear behavior of the stress-strain curve based on the cumulative micro-crack number curves was found to play a vital role in the variations of the degree of brittleness.