In ship and ocean engineering Palmgren-Miners' law is always applied in fatigue life prediction. As to the cyclic stress below the constant amplitude fatigue limit among the variable loads, the stress below the constant amplitude fatigue limit is usually regarded by most classification societies' guidelines that their damage effect can be represented by the constant amplitude S-N curve extrapolated beyond the constant amplitude fatigue limit at a shallower slope, typically m=5 instead of 3. However, many experiments proved that the stress below the constant amplitude probably had more damage than that evaluation result. In this paper, some T-welded joints were chosen as the test object, a list of tests were conducted such as static tests, constant amplitude fatigue tests, and the repeating two-step load block tests with the smaller stress below the constant amplitude fatigue limit. The repeating two-step load block was seldom studied by others. Relative numerical simulation was conducted to predict the potential fatigue spot. The results of the tests showed that the stress below the fatigue limit in repeating two-step load test should not be neglected in fatigue life prediction of T-welded joints and the damage made by the nonlinear effect between two load had more damaging than the damage produced by the small stress range below the fatigue limit in two-step repeating tests.
Ships are prone to fatigue due to the exposure to the cyclic loading environment. The T-welded connection is very common in ship and ocean assemblies. T-welded joints are commonly employed in ship and ocean structures. The fatigue failure of structure components subject to cyclic loading always occurs in T-welded joints due to the metallurgical differences, tensile residual stress fields and stress concentrations. Because ship structures always suffer different kinds of wave loads on the voyage, variable amplitude cyclic loading is the most common service environment of T-welded joint in ship and ocean assemblies (Mansour, A. E., 2003). Stress concentration due to the geometrical shape of weld toe and residual stress as well as inhomogeneous mechanical properties due to the heating and subsequent cooling during weld process lead to most fatigue cracks initiating at the weld toe. Various researches have been carried out with the aim of obtaining a precise and efficient assessment for T-welded joints in ship and offshore structures or proposing a method to improve fatigue property of T-welded joints. (Kim, 2008 Fricke, W, 2002 Healy, B. E, 2004 Gan, J, 2016 and Yildirim, H. C, 2012.) Recently with the high-tensile steel widely used in the shipbuilding industry, some new fatigue problems brings out in the welded toe. In this paper, some experiments were designed in this work with the aim of analyzing the load interaction phenomenon in the variable two-stage load condition. The Q345D hightensile steel, which is widely used in ship and offshore structures, was chosen as research material. The fatigue test was carried out on T-welded component made of Q345D steel. The boundary condition was considered during fatigue test, in addition the leg length of the weld of different specimens were measured and modeled in numerical model with the aim of being more accordant with practical situation.