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

本研究的最终目的是建立焊接结构的设计理念，以防止不稳定断裂。主要进行了以下基础性实验研究：1）基于CTOA R曲线的不稳定十分位断裂研究。实验结果表明，基于CTOA的失稳判据能够较好地预测失稳点。2）对海洋平台结构物缺陷的显著性进行了统计分析。计算了具体构件的断裂概率，分析了参数（如缺陷的可测性、疲劳裂纹扩展速率、韧性的离散性等）的变化对断裂概率的影响。也被检查。通过对T型接头弯曲试件疲劳强度的试验研究，发现对焊接接头疲劳强度影响最大的参数是缺陷的修复能力。4）进行了宽带随机载荷下的疲劳裂纹扩展试验，讨论了随机载荷和等幅载荷下的疲劳行为之间的关系。强调了裂纹闭合对于了解变幅载荷下裂纹扩展的重要性，并提出了随机载荷下疲劳裂纹行为的估算方法。5）实验研究了应变率对结构不锈钢断裂韧性的影响。试验材料为20 mm厚的SUS304钢，试验在液氦温度（4K）下进行。结果表明，应变速率的影响是显著的，马氏体相变的影响也是重要的。

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.