Lee, H., Police, P., Koch, L., Komarivelli, R. et al., "FEA Development of Spot Weld Modeling with Fracture Forming Limit Diagram(FFLD) Failure Criteria and Its Application to Vehicle Body Structure," SAE Int. J. Passeng. Cars - Mech. Syst. 8(1):59-64, 2015, doi:10.4271/2015-01-1316.
Spot weld separation in vehicle development stage is one of the critical phenomena in structural analyses regarding quasi-static test condition, like roof strength or seat/belt pull. It directly reduces structural performance by losing connected load path and occasionally introduces tearing on surrounding sheet metals. Traditionally many efforts have been attempted to capture parent metal ductile fracture, but not applied to spot weld separations in automotive FEA simulations. [1,2,3]This paper introduces how to develop FFLD failure criteria from a series of parametric study on ultra high strength sheet steel and deals with failure criteria around spot weld and parent metal. Once the fracture strains for sheet steels are determined, those developed values were applied to traditional spot weld coupon FEA simulations and tests. Full vehicle level roof strength FEA simulations on a typical automotive body structure were performed and verified to the physical tests.Developed FFLD curves have been used for both spot weld and parent metal and good correlations have been achieved. Various mesh patterns and sizes around spot welds have been investigated. Finally, an optimized weld pattern and mesh size have been suggested for quasi static load cases and successfully applied to vehicle body structures.