Macro-Geometric Defects, A numerical and experimental study of springback and surface defects

Abstract: Today, shortened lead-times in the automotive industry have dramatically increased the need for more efficient development methods at every stage of the process development chain. In order to decrease the long lead-time for producing a forming tool, sheet-metal-forming simulation was introduced. Even though sheet-metal-forming simulation is widely used in the automotive industry today, there are still challenges to be overcome. Two important examples are the prediction of springback and surface defects. If reliable predictions of these phenomena could be achieved, much money could be saved in reduced lead-times and adjustment costs. These areas, together with drawbeads, were also highlighted as areas of significant interest when the use of FE simulation was compared to the use of try-out tools. In this evaluation the Production Performance Matrix (PSM) and the Process Correspondence Matrix (PCM) were used. In order to use the achieved information efficiently, a method for efficient information exchange for the FE simulation results was proposed. The main goal with this project was to find a method for analysis of macro-geometric defects (such as surface defects and springback). Analysis of surface defects involves difficulties in both experimental and numerical assessment. Since the defects are very small and also depend on both position and shape in the classification procedure, it is difficult to find an evaluation system which can detect both small variance in the shape of the surface and classify the defects. Furthermore, the numerical results depend both on the accuracy in the prediction of the forming behavior and the springback. In order to compare the results of the same classification procedure, a method has been developed whereby the surface can be analysed both numerically and experimentally in the same evaluation software. In this way, the classification will be the same and the results will be directly comparable. Regarding springback, a study of the parameters resulted in the conclusion that the important factors to consider in FE simulations of springback are, among others, to have small elements, low tool speed and good material models. The results were applied to an automotive part, a front side member. The simulations exaggerated the twist but the error was moderate for mild steel and Rephos steel. However, a TRIP steel was also examined and there the deviation was much larger. In the forming process the flow of material is controlled among other things by drawbeads. The material flow is very important in order to have a correct stress distribution in the formed part, which in turn affects both springback and surface defects. In this work a method for optimising the restraining force (which represents drawbeads in FE simulations) with a small number of iterations has been developed.