Investigation of Pop-in Formation Mechanism in Fracture Toughness Test of Welded Joint and Improvement of Acceptance Criteria

Abstract

A pop-in is a small, arrested brittle crack that occurs during a fracture toughness test. This is one of the important factors of the CTOD assessment of a welded joint because the fracture toughness of material differs by the significance of the pop-in. The current standard of the pop-in is said to be too conservative because of the reasons below; the absence of the test methodology which ensures the generation of pop-ins, and the difference of the loading mode between the CTOD test and the real structures. The objective of this study is to investigate the mechanism of the pop-in formation and to improve the pop-in acceptance criteria. This study is aimed to come up with a new standard of pop-in significance determination with the idea of the local critical stress criterion and newly proposed crack propagation model incorporating energy consumption theory. That is, the CZM considering that the critical stress works as the local crack propagation criterion, and the fracture surface energy expressing plastic dissipation beneath the running crack wake is defined in each propagation, has been proposed. When the stress of the crack tip approaches the critical fracture stress, the cohesive strength is lost, and the fracture surface is created. When the cohesive strength is lost, it takes away the fracture surface energy. In the physical aspect, the fracture surface energy is increased as the fracture surface roughness increases. The Charpy test with the three-sided specimen is conducted to find the relationship between the fracture surface roughness and the fracture surface energy. The appropriate value or function of the fracture surface energy is determined by the roughness of the fracture surface and trial and error of the FEM analysis. The result of the FEM analysis is well validated with the experimental data. Finally, the authors calculate many extrapolated conditions of toughness distribution and LBZ width and conclude the current pop-in judgment criterion is too severe and propose a new rational one.