A proposed S-N Curve for welded ship structures

Static loads on ship structures induced by cargo loading cause relatively higher stress histories at welded joints compared with cyclic loads induced by waves. Due to these static loads, the initial tensile residual stresses at welded joints are shaken-down to a great extent by the elastoplastic deformation behavior of the material. The redistribution of initial welding residual stresses by the preload was evaluated using finite element (FE) analysis and compared with the results obtained from an ordinary sectioning method for three types of welded specimens, which were all typical fillet weld joints in ship structures. Fatigue tests were performed to evaluate the effects of shaken-down residual stresses on the fatigue strength of the fillet weld joints. The effects of the tensile mean stress on the fatigue strength of preloaded specimens were investigated as well. From the results of fatigue tests, an empirical formula of S-N curves, taking into account the effect of the arbitrary preload and mean stress associated with static loads, was derived based on the hot-spot stress range. The standard deviation between the formula and fatigue test results was calculated. With 2.35% of probability of failure, HD S-N Curve (Hot spot-stress based Design S-N Curve) was proposed.