Study on the safety design of welded structure with respect to unstable fracture

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The final purpose of this study is to establish the design philosophy of welded structures to prevent unstable fructure. The following basic experimental studies are carried out.1) Unstable dectile fracture based on CTOA R-curve in investigated. The experimental results show that CTOA-based instability criterion can predict instability point favorably.2) Statistical investigation on the significance of defects in offshore structures is carried out. Fracture probability for a specific structural member is calculated and the influence of the variations in the parameter ( e.g. datectability of defect, fatigue crack growth rate, scatter of the toughness etc.) is also examined. It is found that the most significant parameter is the retectability of defect.3) Fatigue Strength of the weldment of offshore structure is experimentally analyzed using T-joint type bending specimens. The effect of thickness can be described using elastic stress concentration factor at weld teo.4) Fatigue crack propagation tests under wide band random loading are carried out and the relationship between fatigue behaviors under randam loading and constant amplitude loading is discussed. The significance of crack closure for understanding crack growth under variable amplitude loading is emphasized and an estimating method for fatigue crack behavior under randam loading is proposed.5) The effect of strain rate on fracture toughness of sttuctural stainless steel is experimentally investigated. The material tested is 20 mm thick SUS304 steel and the tests are carried out at liguid Helium temperature (4K). It is found that the effect of strain rate is significant and the effect of martensitic transformation is also important.