Analysis in plastice stage for elastoplastic material

[Lei Lei]

Dear STKOteam,

For elastoplastic materials, different material properties become apparent only after activating the corresponding stage. For example, in PDMY, a value of 0 indicates the elastic stage, while a value of 1 indicates the elastoplastic stage.

Given this, for elastic-plastic materials, is it correct that all loads need to be defined during the elastic stage, with no additional loads applied during the plastic stage, and the system will automatically calculate the plastic effects of the previously defined loads? If so, why do the stresses, strains, and displacements not change in the plastic stage, even though deformation occurs after unloading?

Furthermore, if half of the force from the elastic stage is applied during the plastic stage, the final calculated result will be smaller than if the full force is applied during the elastic stage.

So, for elastic-plastic materials, how to apply load？

Looking forward to your reply!

[STKO Team]

This aspect of OpenSees is a bit confusing.
Some materials for soils need to have an initial pressure to work well without singularities in the plastic regime.
That's why in many examples you see a first load in the elastic regime (to sey up the initial conditions) and then a switch to plastic state.

[Lei Lei]

Some materials for soils need to have an initial pressure to work well without singularities in the plastic regime.
That's why in many examples you see a first load in the elastic regime (to set up the initial conditions) and then a switch to plastic state.

For the initial conditions, all the loads should be applied before the elastic analysis. There should be no additional loads applied between the elastic and plastic state analysis.

In other words, when the analysis switches from the elastic to the plastic state, it will use the loads that were defined prior to the elastic analysis and apply them to compute the response in the plastic state.

Is it right?

[STKO Team]

It's correct.
You can still add extra loads after the plastic stage. They simply do it in the elastic stage otherwise the material won't converge properly in the plastic regime if there is no initial pressure

[Lei Lei]

Thanks for your reply!

Best Wishes!

[STKO Team]

You're welcome