EFFECTS OF STATIC LOAD ON FATIGUE STRENGTH

箱形角焊件存在于船体结构的多个部位，其初始残余应力近似等于焊趾附近材料的屈服应力。利用具有初始残余应力的焊态小试件的试验结果，对焊件的疲劳强度进行了评估。然而，船舶结构构件在投入使用前要经历静载，通常是水舱试验和压载(压载)。采用有限元分析和实际测量相结合的方法，对箱形角焊件的初始焊接残余应力分布和静载预加载时的初始残余应力分布进行了研究。对这些焊件进行了载荷控制的轴向疲劳试验，以考察安定残余应力对疲劳强度的影响。主要内容如下：1)当屈服应力的50%~80%的静拉载荷加载到焊件上时，盒形角焊件的初始焊接残余应力几乎为零，即压应力状态。有限元分析和实际测量都证实了这一点。2)具有稳定残余应力的盒形角焊件的疲劳强度有所提高。3)提出了具有稳定残余应力的盒形角焊件的S-N图。船体结构焊接件的疲劳强度评定应采用曲线图。

Boxing fillet weldments, which exist many parts of ship structure initially have residual stress approximately equal to yield stress of material around weld toe. Fatigue strength of the weldments has been evaluated by using experimental results of small as-welded specimens with initial residual stress. However, ship structural members undergo static loading, usually a tank test and ballast loading (ballasting) before going into service. This static pre-loading is expected to shake-down the initial residual stress.Both the initial welding residual stress distribution on as-welded boxing fillet weldments and the shake-down residual stress distribution on statically pre-loaded weldments were examined by using FEM analysis and actual measurement. Load-controlled axial fatigue tests were performed on these weldments to examine the influence of the shake-down residual stress on fatigue strength.The main points are as follows :1) When static tension load of 50% to 80% of yield stress is loaded onto a weldment, the initial welding residual stress of boxing fillet weldment reduces to almost zero, or compressive stress condition. This has been confirmed by FEM analysis and actual measurement.2) The fatigue strength of boxing fillet weldment with shake-down residual stresses increases. This becomes more dramatic in the longer life range.3) We propose S-N diagrams of boxing fillet weldments with shake-down residual stresses. Diagrams should be applied to fatigue strength evaluation of weldments in ship structure.