Evaluation of fatigue crack propagation in spot welded joints by stiffness measurements

Resistance spot welded joints made of two advanced high strength steels (AHSS) and one high strength steel (HSS) were examined. In the high cycle fatigue regime tests were realised for one loading condition, and the load and the displacement were measured. From the load and displacement range, the stiffness was calculated and in situ normalised to its value after 1000 cycles. It was thus possible to use the normalised stiffness evaluated during the experiment as failure criterion for the fatigue test of resistance spot welded joints. In order to evaluate this failure criterion, fatigue cracks were produced in the three different steel grades mentioned above according to user-specified levels of the normalised stiffness. It was found that the fatigue crack area increases linearly and the through-sheet-thickness crack length exponentially with decreasing stiffness. The comparison of the fatigue crack propagation during the fatigue test of the three different tested base metals showed very similar values. The comparability of fatigue tests performed on spot welded joints might thus be improved by the in situ evaluated stiffness as a failure criterion. This can be also used in further work to verify results of computer simulations of the stress intensity factor at spot welds with the help of experimental results.