Evaluation of specimen geometry effect on brittle fracture resistance based on the local approach

Abstract

The effects of specimen geometry and loading mode on the resistance to brittle fracture initiation were analyzed by the local approach. The fracture resistance of materials under a large scale yielding condition evaluated by the conventional fracture mechanics parameters, such as crack tip opening displacement (CTOD), depends to a large extent on the specimen geometry and loading mode. This is due to the geometrical constraint effect on the crack tip stress field. The three-dimensional FE analysis figured out that, as plasticity developed, a shallow notch bend specimen and tension specimens (CCP and DECP) showed significant loss of constraint compared to a deep notch bend specimen. This led to a lower stress near the crack tip for the shallow notch bend specimen and tension specimens than the deep notch bend specimen. By contrast, the critical Weibull stress at brittle fracture according to the local approach did not depend on the geometry of specimens used. This paper demonstrates the independence of the critical Weibull stress on the crack depth of 3-point bend specimen. As an advantage, the local approach enabled us to predict the specimen geometry effect on the critical CTOD at fracture. The effects of specimen geometry and loading mode on the critical CTOD at fracture depend on the hardenability of materials. In the case of high hardening materials, the critical CTOD is not very sensitive to the specimen geometry and loading mode excepting the thickness effect on the critical CTOD for materials with a large scatter in toughness. On the other hand, for low hardening materials, the critical CTOD is affected to a large extent by the crack depth and loading mode of specimens.