EFFECTS OF STATIC LOAD ON FATIGUE STRENGTH

点击翻译按钮获取中文摘要

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.