new tensor mechanics system should provide mechanical deformation gradient

[hugary1995]

The "mechanical" deformation gradient will exclude "eigen" deformation gradients. Hyperelastic models such as the Simo-Hughes model should use the mechanical deformation gradient instead of the total deformation gradient.

This was discovered in the following discussion:

Discussed in https://github.com/idaholab/moose/discussions/23240

^{Originally posted by **walden37** January 24, 2023} Hi, I have recently been having problems with a thermal mechanical simulation with plasticity. I have implemented plasticity in the old tensor mechanics system through use of the TemperatureDependentHardeningStressUpdate material and in the new system with the ComputeSimoHughesJ2PlasticityStress material. The stresses and strains in the cube differ greatly (approximately ten orders of magnitude) for the two models, despite the eigenstrains almost matching. The model set up is as follows: A cube of length 200mm made of titanium Left face heated to a temperature of 2000K Right face heated to a temperature of 300K Left face of the cubes displacement is fixed in x, y and z The cube has an initial temperature and stress free reference temperature of 1100K The stresses and strains are measured at: measurePoint0=5 measurePoint1=12.5 measurePoint2=25 I have curve fit the PiecewiseLinear hardening functions to use for the Simo-Hughes model. I have attached the input files that produce the different results. I would expect the stresses and strains to approximately match. Could this be a misunderstanding of the Simo-Hughes plasticity model or possibly be a bug? Many Thanks [SimoHughes.txt](https://github.com/idaholab/moose/files/10492270/SimoHughes.txt) [Isotropic.txt](https://github.com/idaholab/moose/files/10492276/Isotropic.txt)

[walden37]

Thanks for the reply, I've attempted to implement the mechanical deformation gradient but it is unclear how this would be done. I have looked in ComputeLagrangianStrain and the total deformation is calculated through the addgradop function acting on the displacements and displacement gradients at each qp. Would it be wise to calculate the mechanical deformation gradient separately ?

[Zoophish]

Any updates on this? Also having issues with Simo-Hughes...

[hugary1995]

There is no update yet. We don't have any funding this fiscal year to support this development. I will have to find some spare time to fix this, or wait until next FY.

[rh201]

hi~ I was wondering are there any updates on this issue? Thanks~