肘板趾端表面裂纹在随机波浪载荷作用下的疲劳扩展预报

Prediction of crack growth of surface crack at bracket toe of ship under random wave loading condition

肘板趾端是船舶与海洋结构的疲劳热点。文章用三维有限元分析了趾端表面裂纹应力强度因子修正系数的变化规律,并与BS7910推荐的典型节点表面裂纹应力强度因子公式计算结果作了对比,结果表明趾端表面裂纹应力强度因子沿深度方向的放大系数和T型节点相差很小,而表面端点应力强度因子修正系数则当裂纹长度在肘板厚度范围内时和T型节点相差很小,超出后则相差较大。以某客滚船上肘板趾端应力范围长期分布服从Weibull分布,产生系列均值为零的应力幅,应力强度因子分别采用有限元结果和BS7910中T型接头公式进行计算,采用单一曲线模型计算该趾端表面裂纹的裂纹扩展。计算等效应力强度因子幅时,考虑焊接残余应力的影响。计算结果表明以T型接头的公式计算趾端表面裂纹应力强度因子和有限元结果相差很小。建议将T型节点表面裂纹应力强度因子计算公式用于趾端表面裂纹应力强度因子的计算,并采用单一曲线模型对随机波浪载荷下作用下船舶典型节点疲劳裂纹的扩展寿命进行了预报。

The bracket toe is one of the fatigue hot spots in ship and ocean structures.In this paper,the SIFs of surface cracks at bracket toe are analyzed by 3D FEA and compared with the results of SIF magnification factors of the same size surface crack at T joints calculated by equations recommended in BS7910.Results show that the SIF magnification factor of surface crack at bracket toe at deepest point is close to that of T-butt joints,while the results at surface point are quite different when the crack length is larger than the bracket thickness.The fatigue crack growth of a surface crack at bracket toe in bottom plate of a Ro-Ro pas-senger ship was predicted by generating series stress amplitude with zero mean stress which the long-term stress range distribution obey Weibull distribution,the stress intensity factors were calculated by FEA results and the T-joint formula in BS7910 respectively,the unique crack growth rate curve model is employed.The welded residual stress effect is considered in the equivalent stress intensity factor amplitude.It shows that the predicted fatigue lives of the SIF of surface crack at bracket toe calculated by FEA and by the T-joint formula are very close.The formula for calculating the SIF of surface crack at weld toe of T joint are recommended to the SIF calculation of the surface crack at bracket toe.The unique crack growth rate curve model is recommended in prediction of the fatigue crack propagation life of weld toe of ship hull under random wave loads.