Fracture Propagation in layered media: A Hybrid Numerical Modeling

Appropriate numerical simulation methods should be selected to model crack initiation and propagation in anisotropic media. Continuum-based finite element methods (fem) are most suitable to model small strain processes within homogeneous media. One would need to use sophisticated functions and some non-physical parameters to implement a limited number of discontinuities or heterogeneity features using FEM. Even then, the nucleation and growth of the failure plane can be modeled on pre-defined pathways only. Discrete element-based methods (DEM) can implement discontinuities and bedding planes as the interface between rigid rock blocks interacting with each other. The global failure plane, however, can grow through the defined interfaces between the large, rigid rock blocks only. To capture fractures through the rock blocks, one needs to adopt the bonded particle method, which makes the computations even more inefficient due to the necessity of tracking particles in addition to blocks.

A better computational scheme would integrate principles of FEM with DEM and take advantage of their strengths, i.e., the efficiency of FEM and the versatility of DEM. The hybrid finite-discrete element methods (FDEM) can capture the transition of a jointed rock mass, consisting of intact rock blocks and closed pre-existing fractures in the natural state, from an initial semi-continuum state to a final semi-discrete state when it is subjected to various perturbations.

In this study, we use FDEM to model the mechanical behavior of a layered rock specimen under tensile, compressive, and bending loads. In this method, layers in a bedded rock sample can be modeled explicitly or implicitly. In the explicit method, mechanical properties of the bedding planes, including the friction coefficient, cohesion, tensile strength, mode I and II fracture energy, and normal and tangential contact moduli of the bedding interfaces would be required. In the implicit method, we can represent the layered sample by assigning five mechanical parameters to the bulk material, including the shear modulus, Young's moduli parallel and normal to the layers, and Poisson's rations parallel and normal to the layers. According to the provided experimental data in the Damage Mechanics Challenge, we will pursue the implicit modeling approach.