Sensitivity to FE modeling parameters

When performing FE calculations the quality of the fatigue life analysis results are affected by the sensitivity to FE modeling parameters of the simplified spot weld representations (such as CDH/SPOT, Nastran/CWELD or simple two-node connections). According to [1], the following parameters need to be considered:

• Mesh density of the shell mesh, adjacent to the spot weld

• Quality of the shell elements (taper, warp, and so on)

• Effective structural stiffness of the spot weld

• Error in the representation of the local geometryInaccurate distance of the sheet mid-planesNon-parallel orientation of the welded surfaces

The first three modeling parameters only affect the NSSA approach. The influence of these parameters is reduced significantly in the DSSA approach, since the local mesh quality at the weld is inherently constant due to the integration of a standardized spot weld representation. The DSSA spot weld representation only allows for variability in the shell mesh in the transition part of the mesh. The stiffness of the DSSA spot weld is also represented with sufficient accuracy.

Local Geometry Errors of Spot Welded Sheets

In the preceding figure, a) shows an inaccurate distance of the sheet mid-planes, and b) shows a non-parallel orientation of the welded surfaces.

But both the NSSA and DSSA approach are influenced by the representation of the local geometry. The preceding figure visualizes both errors in local geometry. The inaccurate distance of the sheet mid-planes influences the lever arm of the joint and has a significant impact on the structural stress at the spot weld. The non-parallel orientation of the welded surfaces leads to errors in the angular orientation of the sheets and can have an effect on the final results. From the start, errors in angular orientation should be minimized as it is very impractical to correct these errors later on.

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Model description—DSSA

Comparison of the results for direct vs. nominal structural stress approach

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