feat: plastic strain output

[ryar9534]

Introducing a plasticStrain output field in addition to the current total strain. This is done by subtracting the elastic strain (obtained via the getElasticStrain method of the constitutive laws) from the total strain and averaging over quadrature points.

Rebaselining will be required because of the new field.

Ideally, plastic strain would only be output when a plasticity model is being used, but Im not sure how exactly we will do that...

I will run the Drucker-Prager and Modified Cam Clay wellbore problem to test

[rrsettgast]

Do we want full tensorial plastic strain, or eps?

[ryar9534]

We must consider the behavior when the following are present:

• Nonzero initial stress (either prescribed or solved for)
• Nonzero initial displacements
• Boundary condition effects

[rrsettgast]

@ryar9534 Can you elaborate? Why do each of these things change the plastic strain calc?

We must consider the behavior when the following are present:

• Nonzero initial stress (either prescribed or solved for)

Is this concern because you have an initial stress in the plastic zone and you don't want to have that plastic strain as part of the reported value? I would think it still needs to be reported as it is the proper current state?

• Nonzero initial displacements

Is this concern that the initial state/strain and the displacement field are incompatible?

• Boundary condition effects

Could we just interpret this as "your BC's are a problem, please use better BC's or move the boundary."?

[ryar9534]

@ryar9534 Can you elaborate? Why do each of these things change the plastic strain calc?

We must consider the behavior when the following are present:

• Nonzero initial stress (either prescribed or solved for)

Is this concern because you have an initial stress in the plastic zone and you don't want to have that plastic strain as part of the reported value? I would think it still needs to be reported as it is the proper current state?

• Nonzero initial displacements

Is this concern that the initial state/strain and the displacement field are incompatible?

• Boundary condition effects

Could we just interpret this as "your BC's are a problem, please use better BC's or move the boundary."?

These were mostly notes to myself to make sure various cases worked as I am figuring things out. Basically the complexity is that the total strain we are outputting is referenced from the initial displacement, but the functions I am using to compute the elastic strain only know the stress. So I was seeing weird results where the two strains are sort of not compatible with one another, and thus you cant subtract them to get the plastic strain.

[rrsettgast]

These were mostly notes to myself to make sure various cases worked as I am figuring things out. Basically the complexity is that the total strain we are outputting is referenced from the initial displacement, but the functions I am using to compute the elastic strain only know the stress. So I was seeing weird results where the two strains are sort of not compatible with one another, and thus you cant subtract them to get the plastic strain.

Oh. Duh. Clearly that is a problem. So you would have to save the initial total strain/initial elastic strain or the initial stress. Nice thing is you could do this one quantity per element rather than per quadrature point.