Sheet metal fatigue near nuts welded to thin sheet structures in car bodies

Paper in proceeding, 2006

The fatigue design process of a car body includes, among others, stress-based sheet metal fatigue near nuts welded to thin sheets (weld nuts). In this investigation, the influence from nut geometry and dimension, and sheet material and thickness, on sheet metal fatigue life was studied by fatigue experiments and numerical analyses. The fatigue experiments were force-controlled and carried out on a weld nut specimen which, during the applied reversed loading, was designed to result in bending fatigue loading conditions of the sheet metal near the weld nut. The results from the experiments were Fa-N curves for various combinations of nut geometry and dimension, sheet material and thickness. Numerical analyses of the experiments were carried out, by linear elastic finite element (FE) analyses, in order to transform the Fa-N curves to Wöhler (σa-N) curves. A simplified FE model of the weld nut was used which is suitable for structural analysis of car body structures. The stresses calculated using the FE model were compared with the stresses calculated using an analytical model that is based on plate theory, and there was good agreement in results between models. The conclusion was that the simplified representation of the weld nut, and the mesh density in the FE model, was appropriate for computation of the stress response near a weld nut subjected to bending fatigue loading conditions. Finally, σa-N curves were computed for each weld nut configuration using its experimental data set. A σa-N curve fitted to all data sets resulted in the relation σa = 5706•N -0.269. The σa-N curves for the eight weld nut configurations tested were gathered within a factor of three in fatigue life from this curve.