Understanding the FLC prediction thanks to fine simulation with damage modelisation

Abstract

Nowadays, FLC is widely used to determine the feasibility of stamped parts. A FLC is usually determined by the experimental Nakajima trials. However, operating conditions can influence the value of the FLC. Experimental trials tend to suffer from result variability (scatters, errors …)and it is rather difficult to interpret the results and find the root causes of such or such phenomenon.The goal of this study is to investigate the influence of operating parameters of the Nakajima test on the FLC by using finite elements simulation. This paper is dedicated to the first phase of this study: define a rather reasonable model that is able to capture the main phenomena. Our goal is not to study and compare very complex plasticity models, that is a very important topic, but also an endless job. We want to build a robust, easy to understand, representative enough model. For that a solid element mesh as fine as CPU time allows was used (0.2 mm) in order to well represent the necking. The procedure used to identify the FLC is the same that for experimental determination. A virtual grid of 1mm on the upper skin is used to compute the strains and the inverse parabola fitting methods of the ISO 12004-2 standard is used to determine the FLC points. Damage was also considered in the model, as it allowed to get closer to analytical and experimental prediction. The damage parameters were a combination of data found in literature and of fitting with an experimental FLC. With this model the effect of thickness was highlighted. Globally it was consistent with what we can observe in a real Nakajima test but we noted a strange behaviour around the uniaxial tension path that need further work to be explained.