XFEM to couple nonlocal micromechanics damage with discrete mode I cohesive fracture
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A computational tool is developed to simulate the propagation of a discrete fracture within a continuum damage process zone. Microcrack initiation and propagation prior to coalescence are represented by a nonlocal anisotropic Continuum Damage Mechanics (CDM) model in which the crack density is calculated explicitly. A damage threshold is defined to mark the beginning of crack coalescence. When that threshold is reached, a cohesive segment is inserted in the mesh to replace a portion of the damage process zone by a segment of discrete fracture. Discretization is done with the extended Finite Element Method (XFEM), which makes it possible to simulate fracture propagation without assigning the fracture path a priori. Rigorous calibration procedures are established for the cohesive strength (related to the damage threshold) and for the cohesive energy release rate, to ensure the balance of energy dissipated at the micro and macro scales. The XFEM-based tool is implemented into an open source object-oriented numerical package (OOFEM), and used to simulate wedge splitting and three-point bending tests. Results demonstrate that the proposed numerical method captures the entire failure process in mode I, from a mesh-independent diffuse damage zone to a localized fracture. Future work will investigate mixed mode fracture propagation.