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The fatigue behaviour of weld ends, especially in thin sheet structures under multi-axial load conditions, has not yet been explored. A research project was initiated to gain more information on this topic which is of special interest to the automotive industry. In the present study fatigue tests on steel tube-tube joints were conducted. Notch stresses were calculated using an idealised weld end model. A numerical method combining the geometrical and statistical size effect into an integrated approach was used in order to take size effects into consideration. The out-of-phase effects on fatigue strength was covered using a multi-axial criterion. We propose a new approach combining the applied multi-axial criterion with the micro-support effect. That leads to a satisfactorily small scatter range of experimentally obtained fatigue lives around the S-N curve from regression analysis. [ABSTRACT FROM AUTHOR]

Many engineering structures are exposed to non-proportional fatigue loading. However, at the critical location of crack initiation, often only one of the load sequences dominates - the individual load cases result in different crack initiation sites. Based on engineering judgment a simplified local uniaxial or proportional loading situation may be assessed. In general, the critical location must be identified by calculating fatigue lives for a variety of locations. The position of the critical location depends on the hypothesis applied for life calculation - besides loading, geometry, material and many other influence factors. For sharp three-dimensional notches the region for the search of the critical location may be restricted considerably. Weld start and stop points are an industrial example for such sharp notches. They are the object of the present investigation. They are exposed to nominal normal and shear loading. The individual hot spots for crack initiation lie necessarily close together. A strong interaction of the loading cases for both proportional and non-proportional loading was experimentally observed. In the numerical investigation notch stresses were calculated using an idealised weld end model. Based on the critical plane approach according to Findley, numerical interaction lines were produced. [ABSTRACT FROM AUTHOR]

ABSTRACT Welding processes often lead to pronounced weld termination points (start/end), which are prone to fatigue events such as crack initiation. The fatigue of weld ends under shear loading, especially in thin sheet structures, is not sufficiently explored yet. In the present investigation, the real geometry of welds was obtained by scanning the three-dimensional surface, especially of weld ends. Cryogenic breaking of the weld opened the view to the weld root geometry and especially to the critical location, where the weld root migrates to a weld toe at the weld end. In the experimental part of this research, fatigue testing of thin sheet plane and cylindrical specimens as well as large components containing weld ends was performed. Based on the data, weld end life curves have been obtained. The notch stress concept and a fracture mechanics based approach were applied for describing the results of fatigue tests providing sufficient accuracy. [ABSTRACT FROM AUTHOR]