Sharp-interface cohesive fracture models with consistent bulk energies: Numerical investigations

We investigate sharp-interface cohesive fracture models formulated as energy minimization problems. We argue that models with arbitrary cohesive interfaces are incompatible with linear bulk elasticity, in the sense that they cannot feature solutions in the form of a regular crack with a simple tip. To this end, we provide analytical and numerical solutions for a model problem consisting of a single straight crack under mode-III loading, where we show that the stress magnitude exceeds the cohesive yield threshold in a finite region around the crack tip. Our findings are consistent with the unavailability of existence results for such models, related to the lack of lower semicontinuity of the associated variational problem. In the mathematical literature, lower semicontinuity and existence of solutions is recovered by introducing a relaxed functional combining the cohesive surface energy on the crack set with a bulk behavior comparable to perfect plasticity, where the bulk strength is determined by the maximal allowable traction of the cohesive law. The relaxed energy provides a homogenised macroscopic model of the possible microscopic structuring of a dense distribution of cracks with vanishing displacement jumps. We report numerical simulations in antiplane shear that illustrate that the relaxed model admits an equilibrium solution in the form of straight cracks that capture both crack nucleation and propagation. Cracks emerging from pre-existing flaws and notches exhibit a smooth transition from classical crack tip plasticity solutions near the notch to a propagating cohesive crack accompanied by an elongated zone around the tip where the nonlinear bulk behavior is active and the stress is constant. We discuss how these observations can inform the development of mathematically consistent coupled models with a minimal number of constitutive parameters, highlighting the inconsistencies observed when arbitrarily combining models with different surface and bulk strengths.