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This paper gives an overview on recent research results of the Darmstadt group inside a project called integral fracture mechanics based determination of the fatigue strength of weldments (IBESS). Purpose of this project is the fracture mechanics based simulation of Wöhler-curves (S-N-curves) of welded joints. Part of this project is the investigation of initial defects and imperfections of welded joints. Metallographic and fractographic investigations have been done to derive a definition of an initial crack situation. Also investigations of the crack growth of multiple short cracks were performed on the basis of thermographic and fractographic studies. Another part of integral fracture mechanics based determination of the fatigue strength of weldments gives attention to the stability of the residual stress field in a welded joint due to the transient plastic deformation behaviour under cyclic loading. Numerical simulations using Döring's material model have been done to investigate the effect of a cyclic loading on a residual stress field in an uncracked and a cracked situation.

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Abstract. The purpose of the present contribution is to predict the fatigue life of welded joints by using the effective cyclic J-integral as crack driving force. The plasticity induced crack closure effects and the effects of welding residual stresses are taken into consideration. Here, the fatigue life is regarded as period of short fatigue crack growth. The node release technique is used to perform finite element based crack growth analyses. For fatigue lives calculations, the effective cyclic J-integral is employed in a relation similar to the Paris (crack growth) equation. For this purpose, a specific code was written for the determination of the effective cyclic J-integral for various lifetime relevant crack lengths. The effects of welding residual stresses on the crack driving force and the calculated fatigue lives are investigated. Results reveal that the influence of residual stresses can be neglected only for large load amplitudes. Finally, the predicted fatigue lives are compared with experimental data: a good accordance between both results is achieved.