Estimation of Fatigue Propagation Life in Resistance Spot Welds

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Abstract
The main objective of this research is to develop a model of fatigue crack propagation in resistance spot welded joints. An important feature of this development is that the model should be usable by designers considering fatigue response of structures containing multiple welds. This objective is achieved by examining the stress state around a load bearing resistance spot weld. A general expression for the structural stress around the weld is formulated that is dependent only upon the loading immediately surrounding the weld and is therefore independent of specimen design. In previous work [1], it was found that structural stress could be successfully used to estimate life for crack initiation and growth to a length of 0.25 mm in resistance spot welds (RSW), and that this period represents less than 30% of the total life in as-welded joints. It is important to note that estimation of this period is highly dependent on fatigue related material properties. In the current work, structural stress is related to crack propagation life through linear elastic fracture mechanics. Life estimates are made using the resulting relationship between structural stress and life. These estimates are compared with experimentally measured lives of welds in HSLA steel. A variety of specimen configurations are considered (e.g., tensile-shear, coach peel) in a number of conditions (e.g., as-welded, pre-stressed). Estimations are all within a factor of three of measured lives. This approach to life estimation uses simple finite element representations of the weld connection to calculate structural stress. It is therefore appropriate for use by designers/analysts making life estimations based on finite element representations of structures containing multiple spot welds.

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