플럭스코아드아크 용접한 API 2W Gr.50 강재의 피로균열전파율의 변동성에 관한 연구

Abstract

Offshore structures and large container ships are received repeated load in their environment. And these structures, generally, consists of large amount of welded components. In assembly of ultra-thick plate, multi-pass welding is in general use. And all the classification societies demand failure design guideline of this weld zone such as fatigue strength and CTOD. There are many welding processes in offshore structure steels. Of those, Flux-cored arc welding (FCAW) is one of the most commonly used methods. FCAW or FCA is a semi-automatic or automatic arc welding process. This process is widely used in offshore and ship building industry because of its high welding speed and portability. Most of the components of marine and offshore structures are subjected to cyclic loading during their service time due to action of seawater waves and the sea environment in general. Fatigue failures in these structures can take place at sites of high stress concentration that can be classified into two major categories, that is base material and weld joints. Welded joints can contain variation type of flaws slag inclusions, and gas pore, etc.. From such flaws the service period, a crack can initiate, grow slowly and finally lead to catastrophic failure. The process of fatigue crack initiation and fatigue crack growth is uncertain by nature, due to variability of material properties, loading fluctuations, geometrical properties and the environmental conditions, such as temperature and humidity. For practical application, it is important to understand the fatigue crack growth behavior of cracked welded structures. Fatigue crack growth rate data of welded structures should be determined accurately so that the 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. However, 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. It is essential that the procedures are reliable and sufficiently conservative. The objective of this study is to investigate the randomness of fatigue crack growth rate (FCGR) for the FCAWed API 2W Gr.50 steel weldment tymically applied for offshore structures. In order to investigate clearly the randomness of fatigue crack growth rate for the materials of three different zones WM, HAZ and BM, experimental fatigue crack growth tests for each five specimens were performed on ASTM standard compact tension (CT) specimens under constant amplitude cyclic loading. It was shown that the randomness of FCGR for WM specimen was higher than those of HAZ and BM, and the probability distribution of FCGR was followed well Weibull distribution.