Analysis of Crack Initiation and Propagation Mechanisms in Polycrystalline Rocks Using Discrete Element Modeling

Discrete element modeling has proven highly effective for understanding the failure mechanisms in polycrystalline rocks by simulating their microstructure through a deformable polyhedral grain-like model. However, calibrating these models by adjusting micro-parameters presents challenges, especially across samples of varying dimensions. Therefore, the present study employs a novel constitutive model where contact behavior is controlled by contact cohesion (c), contact friction (ϕ), tensile strength (ft), joint normal and shear stiffness (jkn, jks), fracture energy in tension, shear, and compression (Gf1, Gf2, Gc). Utilizing this model, the study examines the effect of various particle size distributions on the macro-behavior of models. The failure characteristics of Jodhpur sandstone, a homogeneous and isotropic rock, when subjected to Uniaxial Compressive Strength (UCS), Brazilian Tensile Strength (BTS), and Semi-Circular Bending (SCB) tests are simulated. The sandstone models are discretized into constant-strain tetrahedral elements, incorporating both tension-shear and compression-shear interactions at the contact interfaces. An in-depth examination of sub-contact states offers insights into contact point interactions and the evolution of effective stresses and block displacements under progressive shear and tensile deformation. The proposed constitutive model within the DEM framework replicates laboratory-observed failure behaviors, accurately modeling shear failure in UCS tests and crack width and its propagation in BTS, fracture toughness, and crack mouth opening displacement CMOD in SCB samples. The study reveals that reducing the number of grains significantly decreases compressive strength, tensile strength, and the modulus of elasticity (E). This model offers significant potential for analyzing rock failure in engineering scenarios, such as tunnel excavation in rock masses, where similar failure mechanisms are encountered.