DEM investigation of the crack effect on the behavior of brittle granular materials subjected to one-dimensional compression

Granular materials have micro cracks in their structure due to changes in temperature, pressure, and weathering. These microcracks, which are distributed within the grains in different lengths, directions, and positions strongly affect the mechanical behavior of grains such as stiffness, strength, and breakage. On the other hand, the discrete element method is a powerful tool for the analysis of granular materials. Ability to model different types of grain shape, loading conditions, and cracking in materials are among the features of this method. Therefore, by modeling cracked grains by discrete element method, the effect of cracking on material behavior can be evaluated. In this paper, cubic and cylindrical cracked and non-cracked grains are modeled and subjected to uniaxial loading with lateral confinement. Using Hertz nonlinear contact model, performing sensitivity analysis to determine the minimum number of balls required to form each clump, controlling the number of contact points, slope and direction of cracking plates in cracked grains to ensure their uniform distribution in different modeling and using the combined criterion of tensile strength and fracture toughness in terms of combination modes of one and two are among the features of this numerical model. Following the validation of the numerical model with similar laboratory results and ensuring the operation of the model, at this stage, to investigate the effect of crack direction on the behavior of materials, cracked grains are regularly placed on top of each other and at each stage of loading, the direction of the cracks changes from zero (parallel to vertical force) to 90 degrees (perpendicular to vertical force). Finally, the combined arrangement of cracked and non-cracked grains at different ratios is modeled and their behavior is evaluated. The results show 16% and 21.5% increases in applied energy and 19% and 6% increases in strain values, respectively, in cracked cubic and cylindrical specimens. Moreover, the breakage factor increases almost 12% in cracked specimens. The effect of crack inclination at a 45-degree angle is maximal so that the fracture stress is 17% smaller than the average fracture stress at different angles. Finally, for any other desired combination of cracked and non-cracked grains, for a given stress, the amount of breakage factor and the corresponding strain in this range can be estimated through numerical modeling.