| Starting with the pioneering work of Dugdale and Barenblatt, cohesive failure models have been widely used in the analytical and numerical study of quasi-static and dynamic fracture problems. However, some work remains to be done to understand fully the effect of the parameters defining the failure model on the mechanics of dynamic fracture under both steady-state and transient conditions.; In this thesis, cohesive failure modeling is applied to two distinct dynamic fracture problems. The first one is concerned with the dynamic propagation of cracks on a rate-dependent material. More specifically, the objective is to understand how the introduction of rate dependence in the cohesive model affects the fracture process. The analysis is performed using a Cauchy integral formulation in the steady-state case and a spectral form of the boundary integral form of the elastodynamic equations in the transient case. Various extrinsic and intrinsic cohesive models are investigated. The analysis shows that limiting (quasi-steady-state) crack velocities less than the shear (Rayleigh) wave speed are possible under spontaneous propagation conditions. Due to the rate-dependent dissipative mechanism in the cohesive zone, the predicted fracture toughness increases with the crack speed.; The second dynamic fracture problem addressed in this thesis is concerned with the issue of intersonic crack propagation, a topic of great interest in the fracture mechanics and geophysics community with the recent first direct observations of shear cracks exceeding the shear wave speed in homogeneous brittle specimen. Special interest is placed in this study on the possibility of steady-state and transient intersonic crack motion under mixed-mode conditions. Results show that exclusive shear damage in the cohesive zone is responsible for intersonic crack propagation even though the external load comprises of both shear and tensile components. The situation is very different in the subsonic regime, where crack motion is a result of combined shear and tensile damage inside the cohesive zone. The study also provides information on the effect of mode mixity on the length of the cohesive failure zone. |
