Loading and Un-Loading of a FE Model for ElastoPlastic Analysis

[struclearner]

The Stress values of a model with plasticity, material defined with a stress strain data, after yielding does not shows zero stress value after un-loading, although the elastic strain values goes to zero and plastic strain values is representing the Hysteresis loop.
At the zero load, the time history plot of stress should show zero stress.
The analysis of a small test model is done with Load step file, the first step is loading the model and the second step is un-loading the model with zero load applied.
The load applied is causing yielding and there are plastic strains at the notch of the model.

Thanks for input/help.

 

[FEA way]

Well, the first thing that comes to my mind is that residual stresses might have developed in your model. It's quite normal since it happens in many physical processes.

 

[struclearner]

Yes, this is true, but what input for the FE analysis, is causing the program, to calculate the residual stresses?
The material model used in the analysis is Miso, with stress strain curve used.

 

[Erik Panos Kostson]

I use this example when teaching nl fea.

Pipe with internal pressure above yield so the inner radius goes plastic. Unload that and the inner radius will go into compression. This is because outer radius material is elastic trying to go back during unloading thus squeezing inner radius into compression. This can be used to increase fatigue resistance.

This is also a good example:

https://www.comsol.com/blogs/model-residual-stresses-using-comsol-multiphysics/