On the determination of the cohesive zone parameters for the modeling of micro-ductile crack growth in thick specimens

Abstract

The paper deals with the determination of the cohesive zone parameters (separation energy, Gamma, and cohesive strength, T-max) for the 3D finite element modeling of the micro-ductile crack growth in thick, smooth-sided compact tension specimens made of a low-strength steel. Since the cohesive zone parameters depend, in general, on the local constraint conditions around the crack tip, their values will vary along the crack front and with crack extension. The experimental determination of the separation energy via automated fracture surface analysis is not accurate enough. The basic idea is, therefore, to estimate the cohesive zone parameters, Gamma and T-max, by fitting the simulated distribution of the local crack extension values along the crack front to the experimental data of a multi-specimen J(I C)-test. Furthermore, the influence of the cohesive zone parameters on the crack growth behavior is investigated. The point of crack growth initiation is determined only by the magnitude of Gamma. Both Gamma and T-max affect the crack growth rate (or the crack growth resistance), but the influence of the cohesive strength is much stronger than that of the separation energy. It turns out that T-max as well as Gamma vary along the crack front. In the center of the specimen, where plane strain conditions prevail, the separation energy is lower and the cohesive strength is higher than at the side-surface.