Dear researchers,
Currently I am analysing data from a forward creep test FE model. And in my results during uniaxial creep under constant load, I see that there is a plastic strain increase as the specimen creeps.
After loading the peak stress in the gauge section is 408 MPa, as specimen creeps, the stress increases by 1.5% due to reduction of area over the course of time step. By looking at the plastic strain vs time, can be seen that plastic strain increases during creep step.
Does additional plastic strain come from stress relaxation in other parts of the specimen or I am missing something ?
P.S. Specimen geometry attached for illustration purposes.
Dear Janis,
I actually say this "mechanism" occurring in some of my simulation as well. I defined some criteria and I was writing a research paper on this phenomenon that is very difficult to see in experiments where the creep strain and plastic strain cannot separated. What I understood is that the stress relaxation in your component tends to create a residual stress field in the section. This while the specimen still under load can potentially lead to an additional plastic strain. In some cases this is very small and can be neglected. But in many other it represents a problem, especially if the applied load is cyclic.
What material model are you using? EPP, Chaboche, soemthign different?
Thank you for your answer Daniele.
For my material model I am using stress-strain curve from experimental data together with simple norton creep law.
I think i will run a simple model of an axially loaded bar to see , if there is additional plastic strain accumulation for a structure with uniform width.
I think this phenomena becomes more relevant at higher stresses, lets say, if material is loaded close to UTS. Even small increases in stress due to relaxation can lead to higher accumulated plastic strains.
Janis,
In general, when a smooth specimen is modelled (round or flat), von-Mises and maximum principal stress will remain the same unless a damage criteria is incorporated in the material model (I assume you have applied constant distributed load- or pressure/stress). However, If any discontinuity is there, there is a possibility of stress relaxation. Try to model the geometry without grips (i mean only the gauge length). Hope this problem will not arise.
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