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Overview on local concepts for the fatigue design of welded structures
Recently developed new variants of structural stress, notch stress and fracture me-chanic concepts are discussed. The two new structural stress concepts described first are based on a linearization of the stress distribution over the thickness or over the anticipated crack depth and on the local stress 1 mm below the weld toe. Their aim is to define the structural stress and to evaluate it in respect of an appropriate Woehler-curve. A further structural stress concept is presented for thin steel and aluminium welded sheets. For the evaluation of the calculated struc-tural stress, loading mode (axial or bending) dependent master Woehler-curves are proposed.
For notch stress concepts the newly verified one with the fictitious radius ?f = 1 mm for welded aluminium with a thickness higher than t = 5 mm and the one for thin sheets with substitute radius ?f = 0.05 mm are outlined. The second concept is applicable to spot welds too. For spot welds, also another local concept basedon elastic-plastic notch strains is demonstrated, which delivered encouraging results.
The two fracture mechanics concepts (conventional crack propagation and NSIF) for spot and seam welds allow to describe the local phenomena more realistically and reduce the scatter of the correspondingly evaluated fatigue life more than the structural and nominal stress concepts.
For cumulative damage calculations the use of the damage sum D = 0.5, and for the evaluation of multiaxial stress states the use of the Gough-Pollard-ellipse with load case and material de-pendent damage sums is recommended. Both methods are applicable for the nominal, structural and notch stress concepts.