Spot weld fatigue life analysis

Introduction

Developments for dedicated fatigue analysis of spot welded connections have mainly been driven by the need from the automotive industry. The reason for this is that the durability behavior of the automotive steel body structure is dominated by the fatigue of the spot welds, used in the creation of this body-in-white (BIW). To support the BIW design validation many automotive OEMs perform finite element based numerical fatigue life analyses of the body-in-white. Therefor it is essential that the available CAE simulation methods have dedicated spot weld fatigue life analysis approaches in order to allow the prediction of lifetime to crack initiation of spot welded configurations with sufficient accuracy.

Over the years multiple approaches have been developed and published by different researchers. However, only two approaches have implemented in today’s commercially available software packages: the force based approach of Rupp et al. [2, 3] and the stress based approach, presented by Brenner et al. [4].

Both methods are based on a local concept that uses structural stresses, as introduced by Radaj [5]. The force based approach of Rupp makes use of analytical engineering formulas to back-calculate a structural stress parameter from sectional forces and moments that act in a simplified FE representation of the spot weld. The stress based approach of Brenner attempts to implement Radaj’s concept directly in the structural analysis, but at the cost of a significantly coarsened shell mesh (8-element ring surrounding an octagonal hybrid beam/shell nugget representation). The coarsened shell mesh leads to a high degree of stress averaging in a region of large stress gradients. [1].

Radaj's Concept for Spot Weld and Various Implementations Including NSSA and DSSA

The described Simcenter 3D approach is based on a finite element simulation procedure that exploits Radaj’s original concept directly in the structural analysis procedure without introducing any simplifications, assumptions or submodeling techniques. This means that the structural stresses, relevant for the fatigue calculation, are directlyderived from a structural finite element model that has an appropriate numerical discretization of the spot weld nugget. This approach is referred to as Direct Structural Stress Approach or DSSA, while the force based approach of Rupp and co-worker’s is also referred to as Nominal Structural Stress Approach or NSSA. shows the various implementations, based on the Radaj concept for spot welding. Rupp’s approach (NSSA) can be found in the top-left corner, while Brenner’s simplified stress approach is display in the top-right corner. The DSSA approach can be seen at the bottom of the figure above. The NSSA and a simplified version of the DSSA method are available in Specialist Durability. The detailed DSSA method can be implemented in Specialist Durability starting from the simplified DSSA method.

The DSSA development was motivated to overcome practical limitations of overall predictive accuracy, observed when using the NSSA method for congruent and non-congruent meshes. [1].

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Possible failure modes for spot welds

Spot weld fatigue life analysis bibliography

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Spot weld fatigue life analysis, Simcenter 3D 2021.1 Series

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