Effects of the cohesive law on ductile crack propagation simulation by using cohesive zone models

The cohesive zone model has been applied in different computational fracture mechanical investigations. However, effects of the cohesive law on crack simulation results have not been systematically and quantitatively studied. To quantify the influence of the cohesive law, a special cohesive element has been developed and implemented into the commercial FEM code ABAQUS. The detailed computational investigation of compact tension (CT) specimens confirms that the load vs. load line displacement curve hardly depends on the initial cohesive stiffness of the cohesive zone, but the fracture parameters, such as δ5, may deviate up to 5%. A significant difference is observed in prediction of crack propagation, which exceeds 35% for a given load line displacement in CT specimens. To diminish artificial influence of the cohesive zone model, one has to increase the specific cohesive stiffness. The J-integral as the critical energy release rate generally differs from the cohesive energy. The elastic unloading and plastic reloading around the cohesive zone affect the fracture energy amount. The difference between the cohesive energy and the critical energy release rate depends on the cohesive law as well as the ductility of the material, vanishes only in an elastic specimen and exceeds 40% for ductile materials. The discrepancy between J and the cohesive energy grows and is stagnated for the cohesive strength larger by three times the initial yield stress. To obtain realistic computational results by using cohesive zone models, one has to build the cohesive law with proper parameters.