Development of a three-dimensional fracture model for the combined finite-discrete element method

Abstract

Three-dimensional fracture simulation is a big challenge to computational mechanics because of the complicated fracture surface geometry and the difficulty of characterising different failure and interaction mechanisms in complex three-dimensional stress fields. As an attempt to improve the ability of numerical methods to simulate discrete fractures in quasi-brittle materials, a three-dimensional fracture model is developed in the context of the combined finite-discrete element method. The proposed fracture model is capable of simulating the whole fracturing process for both tensile and shear fracture initiation and propagation, including pre-peak hardening deformation, post-peak strain softening, transition from continuum to discontinuum, and explicit interaction between discrete fracture surfaces. An adaptive remeshing algorithm is developed to simulate discrete fractures with less mesh dependency. This algorithm can accurately refine tetrahedral elements based on the local stress field, and update the local mesh as fractures propagate. As a further development of the fixed-mesh-based fracture model, it is incorporated into a two-way fluid-solid coupling model and it is successfully applied to simulate a hydraulic fracturing problem. A comprehensive numerical simulation is carried out by applying the proposed three-dimensional fracture model to investigate explicit fracture development and to evaluate the damage mechanisms of concrete armour units on breakwaters. Dolosse units are simulated in drop tests and pendulum tests. The dropping of an assembly of CORE-LOCTM units of prototype scale is simulated under an imaginary extreme loading condition. The whole structural response of the CORE-LOCTM units is accurately captured and the transient dynamic response including that by fracturing is explicitly characterised. To investigate fracture network formation and growth in multi-layered rock, two-dimensional and polyaxial deformation simulations are conducted. Three-dimensional stress heterogeneity caused by fracturing is accurately captured. The results show the three-dimensional fracture model is capable of generating realistic fracture patterns according to geomechanical principles of rock failure.