Wrinkling prediction in sheet metal forming and experimental verification

In this work, the analysis of Hutchinson and Neale is used for wrinkling prediction. Under a number of assumptions, limitations and simplifications a wrinkling criterion with some restrictive applicability, is obtained. Unfortunately, Hutchinson analysis is limited to regions of the sheet that are free of any contact. When contact is taken into account the problem is further complicated. Consequently, a local indicator based on the change of curvatures under compressive stresses is developed. Both wrinkling indicators are used to drive the adaptive mesh refinement in order to be able to accurately spot wrinkling. The numerical results will be compared to those obtained through experimental testing. A number of hemispherical product samples have been used with various blank holder forces and drawn to different depths to capture the onset of wrinkling, its mode and locatio

Characterisation and modelling of the plastic material behaviour and its application in sheet metal forming simulation

The application of simulation models in sheet metal forming in automotive industry has proven to be beneficial to reduce tool costs in the designing stage and for optimising current processes. Moreover, it is a promising tool for a material supplier to optimise material choice and development for both its final application and its forming capacity. The present practice requires a high predictive value of these simulations. The material models in these simulation models need to be developed sufficiently to meet the requirement of the predictions. For the determination of parameters for the material models, mechanical tests at different strain paths are necessary 1. Usually, the material models implemented in the simulation models are not able to describe the plastic material behaviour during monotonic strain paths sufficiently accurate 2. This is true for the strain hardening model, the influence of strain rate and the description of the yield locus in these models. A first stage is to implement the improved material models which describe this single strain path behaviour in a better way. In this work, different yield criteria, a hardening model and their comparison to experiments are described extensively. The improved material model has been validated initially on forming limit curves which are determined experimentally with Nakazima strips. These results will be compared with predictions using Marciniak-Kuczinsky-analysis with both the new material model and the conventional material model. Finally, the validation on real pressed products will be shown by comparing simulation results using different material models with the experimental data. The next challenge is the description of the material after a change of strain path. Experimental evidence given here shows that this behaviour cannot be treated using the classical approach of an equivalent strain as the only history variable