Multi-axial fatigue of welded details

Licentiatavhandling, 2012

Welds and welded joints are nowadays very common in steel structures. Since the welded parts also are susceptible to fatigue, they can have a major influence on the total life of the structure. Crack growth in a weld, which can eventually lead to failure, can occur in two principally different ways namely at the toe, where the crack propagates into the base material, or in the root, which means that the crack grows through the weld itself. A considerable amount of research has been devoted to determine the fatigue strength of various welded details under multi-axial stress conditions but the focus has primarily been on toe failure. The number of investigations on root failure is considerably less. This thesis presents new fatigue data for a welded detail that fails in the root and which is subjected to a multi-axial stress state, simultaneously acting normal and shear stresses. A significant difference in the fatigue strength could not be found based on the ratio between shear and normal stress, indicating that shear stress does not have a large effect on the fatigue strength of the examined detail. When assessing the fatigue life of a detail it is important to calculate the stresses correctly. The nominal stress method takes into account only the geometrical changes that affect the global stress in the cross section such as the making of holes. However, over the years more advanced methods have been developed to more accurately calculate the stress state in a detail, for example the hot spot method and the effective notch stress method, which are based on the finite element method. The use of these methods is associated with a considerably increase in work effort. A comparison of the three methods was made within the framework of this thesis. It was further investigated if there is something to gain in choosing a more advanced method compared with the nominal stress method, especially if the increase in work effort is taken into account. The results indicate that for the studied details there is no obvious advantage in doing this. However, there are cases where the nominal method is not applicable and by that the advanced methods are the only alternative. Key words: Welded details, multi-axial fatigue, throat failure, hot spot stress method, effective notch stress methods