Sheet metal fatigue near nuts welded to thin sheet structures

Artikel i vetenskaplig tidskrift, 2008

One of the fatigue design processes of a car body is stress-based sheet metal fatigue near nuts welded to thin sheets (weld nuts). In this investigation, the influence from nut geometry and dimension, as well as 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 reversed loading that was applied, 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 (sig_a–N) curves. A simplified FE model of the weld nut, which is suitable for structural analysis of car body structures, was used. The structural stresses calculated using the FE model showed good agreement with the structural stresses calculated using an analytical model that is based on plate theory. The conclusion was that the simplified representation of the weld nut, and the mesh density used in the FE model, was appropriate for computation of the stress response near a weld nut subjected to bending fatigue loading conditions. Finally, ra–N curves were computed for each weld nut configuration using its experimental data set. A sig_a–N curve fitted to all data sets resulted in the relation sig_a = 9888N^0.312. The sig_a–N curves for the 21 weld nut configurations tested were gathered within a factor of three in fatigue life from this curve.