Fatigue of Spot Welded Joints: Experiments and Life Predictions

Doctoral thesis, 2001

Methods to assess the fatigue strength of spot welds in built-up structures are used on a daily basis in the automotive industry. Efficient and reliable numerical methods make it possible to achieve designs with low weight, low manufacturing costs and good durability within short development cycles, since the number of prototypes built can be reduced. To refine the existing methods further, this thesis investigates the fatigue behaviour of spot welded joints. Experiments as well as three-dimensional elastic and elastic-plastic finite element simulations were carried out to investigate the influence of welding residual stresses, spatial variation in yield stress, and fatigue properties on fatigue crack initiation at spot welds. It was found that tensile residual stresses relax for higher loads, and that linear elastic fracture mechanics could be used to predict the early growth of fatigue cracks initiating at the spot weld perimeter. The fatigue crack propagation through the sheet thickness at peel and shear loaded spot welds was then studied experimentally with interrupted fatigue tests. These tests showed that fatigue life was much shorter for a shear loaded weld than for a peel loaded one when both were subjected to the same initial stress intensity factor range. This was taken into account by a new fatigue life prediction method for built-up structures. A new shear correction factor takes the crack propagation into account, although only the initial stress intensity factor range is used for the fatigue life prediction. The method was verified with fatigue data from eight specimens. The effects of mean stress on the fatigue life of peel and shear loaded welds were investigated experimentally. The results observed could be explained by crack closure. This was found both experimentally and with elastic-plastic finite element simulations. Fatigue tests with variable amplitude loads were also conducted. It was found that the Palmgren-Miner cumulative damage rule can give non-conservative results by up to a factor of four, because of load interaction effects. To avoid this, a new fatigue life prediction method for variable amplitude fatigue of spot welds was developed and verified experimentally. The method is based on crack closure and linear damage accumulation.