Investigation of factors affecting cracking during forming of truck cab body parts

Abstract: Sheet metal forming is a technique widely used in todays industries as it enables fast transformation of metal sheets into parts of various shapes and sizes. Volvo GTO EBM in Umeå uses sheet metal forming in their production of truck cab body parts, and a challenge common for all industries using this technique is to avoid cracking in the formed metal. The present study has been conducted in order to increase the understanding of why produced articles sometimes crack, so that cracking can be prevented in future production. This has been done by studying how different variations in the production process are affecting the robustness of produced articles, partly by investigating variations in material properties and partly by running robustness simulations of a chosen article prone to crack. Material properties have been gathered for both cracked and non-cracked details of different articles, and thereafter been compared to each other using multivariate analysis. Furthermore, simulations have been run using the software AutoForm with the purpose to investigate factors such as feeding direction of metal sheets, variations in material properties and different forces and velocities used during forming. Experiments have also been conducted in order to compare the simulation model and simulation results with reality. From the material property analysis it could be seen that differences in material properties do exist between cracked and non-cracked materials, but that these differences vary between different articles. The robustness simulations indicated that a certain force called draw cushion force do affect the final robustness of an article the most, compared to the other affecting factors investigated in this study. Moreover, the simulation set-up did seem to agree with reality, while the simulated material thinning deviates more than 20 \% from the real one. This may result from an inadequate modeling of friction in the simulations. In conclusion, material properties are important concerning cracking during sheet metal forming, and the draw cushion force seems to have the strongest influence on article's final robustness.