Establishment of damage estimation rules for brittle fracture after cyclic plastic prestrain in steel

Abstract

The microprocess of brittle fracture in steel is suggested to occur because microcracks appearing in the brittle layer at the grain boundary propagate driven by increased piled-up dislocation. Thus, loading plastic deformation, equivalent to the increase in dislocation, is directly connected to the increase in brittle fracture risk. Because of these mechanisms, prestraining means approaching the limits of the material, which could lead to deterioration of fracture toughness. For a lifetime evaluation of steel structure, it is important to generalize the effect of various cyclic prestrains.In this paper, material toughness changes when tensile and compression prestrain occur were investigated. A general three-point bending test was employed to evaluate fracture. Synthetic analysis using Conventional Mechanism-based Strain Gradient Plasticity (CMSGP) theory, in which dislocation density is used to determine the amount of material damage, was carried out as well as employing conventional macroscopic material damage consideration rules. The consistency of the simulation and the experimental results were supported by the Electron Back Scattered Diffraction Pattern (EBSD) observation. Additionally, critical stress was calculated, and the change in critical stress for prestrain conditions was investigated. Then, the mechanism of critical stress changes could be expressed by the yield point increase and dislocation density. Keywords: Material damage, Prestrain, Conventional mechanism-based strain gradient plasticity, Critical stress, Dislocation density