다층 FCA용접한 해양구조물용 강재의 피로균열전파저항에 관한 연구

Abstract

Abstract

Fatigue cracking of structural details in ship and offshore steel structures due to cyclic loading has gained considerable attention in recent years. Numerous research works have been conducted in this field on both theoretical and practical aspects. Consequently, a great deal of studies have been published resulting in various topics relating to fatigue crack initiation and propagation problems. Welding is most widely utilized as a joining method for steel structures. Among many welding processes, the Flux Cored Arc Welding (FCAW) process has grown in use in recent times as a function of its characteristics of high productivity, good quality and low cost, associated with suitable mechanical properties in welded joint, especially in ship and offshore structure fields. However, the welded joints often contain defects such as slag inclusions, incomplete fusion, gas pores, undercuts at weld toes, etc. The welded joint consists of three zones from the material's microstructural viewpoint : weld metal (WM), heat affected zone (HAZ) and base metal (BM). The individual zones exhibit different mechanical behavior such as hardness, and fatigue properties. Several researchers have revealed that no significant variations exist in the fatigue crack propagation properties for the three zones, however some others showed that these three zones had significant differences in fatigue crack propagation behavior. For practical application, it is important to understand the fatigue crack propagation behavior of cracked welded structures. Fatigue crack growth rate data of welded structures should be determined accurately so that the fatigue crack propagation life of the welded structure can be accurately evaluated. Several fatigue crack growth data for the welded joints are available but very few data are available for the welded structures in the stress range viewpoint. And also, it is widely recognized that the fatigue crack growth is fundamentally a probabilistic phenomenon. Several studies have demonstrated the inherent randomness of fatigue crack growth beyond crack initiation. Risk assessment and remaining fatigue crack growth life prediction require accurate probabilistic fatigue crack growth models. Therefore, there are tow main aims in this study. The first one is to investigate the effects of the load range on fatigue crack growth behavior in three different zones, WM, HAZ and BM for API2W Gr. 50 steel. The second aim of this study is to investigate the effects of the load range on the spatial variation of fatigue crack growth resistance in three different zones, WM, HAZ, and BM for API 2W Gr. 50 steel using the stochastic model based on reliability theory. Experimental fatigue crack propagation test were performed on ASTM standard CT specimens. The results indicates that the load range have strong dependency on fatigue crack propagation for the three different zones WM, HAZ and BM, and also, on spatial variation of fatigue crack propagation resistance.