In my opinion Von Mises stress which is just a scalar value of the combination of the three principal stresses tells you which regions of the structure have started yielding(you need to have a experimental value to compare for this). So in your structure, though its starting to show plastic deformation doesnt mean that all of a sudden the whole structure is in the plastic domain,rather its a process where various points will fall into the plastic regime depending on the stress and time steps that you have. Remember that stress/strain is only calculated at the gauss points(integration points) and for each gauss point you have a different stress strain curve thus some points will go into the plastic regime earlier than other points.

Hi A. Wasay Khan. Thanks for your reply. As I mentioned, I am doing a metal forming simulation where the entire structure is continuously undergoing plastic deformation. It is well beyond it’s elasttic limit since it needs to be formed permanently into a totally new geometry.

So my question is, in this case where I know it will be undergoing large plastic deformations, why would I care about the stress distribution?

Did you start the forming from a sheet? If it was, perhaps the Mises stress represented the deformation degree, which was converted from computed strains through a equation such as Swift expression, Ramberg-Osgood expression etc.

Hi Gyoko, that’s exactly right. The structure prior to deformation was a sheet and the material model was the Ramberg-Osgood equation. In my model, i have modelled the cross section of the structure, to investigate the plastic deformation behaviour through the section. I am interested in the Equivalent plastic strain distribution through the section, however I am not sure what benefit the Mises stress (or any stress) gives me. Since i know the structure is already undergoing fierce plastic deformation.

Could you please tell me if looking at the stress has any significance in this case?

Hamid, perhaps the Mises stress can be used as a reference indicator for the breakage occurring in the forming process and where it easily is.

Whether you should care about the von Mises Stress depends on what you are actually trying to calculate. PEEQ might be a more important variable, depending on what you are doing. What is the goal of your simulation?

Hi Martin, thanks for your answer. PEEQ is certainly of interest since I am trying to understand the plastic deformation through the thickness of the material (hence why I have modelled the 2d section).

In what scenario would Mises Stress be useful?

Hey Hamid, correct me if i'm wrong, but i can imagine the MISES stress being important in case you would want to simulate this workpiece with the influence of residual stresses due to the forming process. In this case the stress Distribution would be the initial state of your Simulation and failure of the workpiece would strongly depend on the prestress distribution.