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

Abstract

A pop-in is a small, arrested brittle crack 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 a 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 specimen which ensures 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. Firstly, experiments are carried out with the “brittle bead specimen” which is a welded specimen that artificially represents the local brittle zone so that the pop-in stably occurs. From the result of the experiment, the standard of the load drop ratio of the pop-in can be set as 10% from the current 5%. Next, the finite element method analysis is performed to see the difference of the loading mode of the bending and tensile. From the result of the FEM analysis, it is suggested that the previous research of the pop-in is incorrect and the new model of the pop-in formation with the parameter of the width of the local brittle zone is proposed.