용접부 CTOD 시편의 외력에 따른 잔류응력 재분포에 관한 연구

Abstract

As the one of the fracture toughness tests, CTOD test is an important factor to ensure resistance to fracture at severe conditions in designing offshore structures. In this study, to obtain fracture toughness values of thick welded steel plate, CTOD test was conducted. When conducting CTOD test, fatigue pre-cracking occasionally failed to satisfy the requirements of standards making the test result invalid. The internally accumulated residual stress of CTOD test piece has been thought as one of the main reasons. The propagation of fatigue crack started from the tip of machined notch, which might have grown irregularly due to residual stress field. To overcome this kind of difficulty, in BS 7448 Part 2, Local compression, reverse bending, and stepwise high-R ratio methods are suggested to modify the internal residual stress. In this paper not only multi pass welding using FCAW(Flux Cored Arc Welding) but also Local compression process of thick steel plate has been simulated using the finite element method to clarify variations of internal welding residual stress. The simulated results show that welding residual stress is compressive in the middle section of the model. After machining the specimen, tension residual stress predominantly increased in the middle section of the model. Through these variations of residual stress, we can anticipate the propagation of the fatigue crack. Analysis results show that this irregular welding residual stress distribution is improved to be more uniformly by applying Local compression method.