Negative plastic moment

[NicoSchlw]

Describe the bug
The plastic moment output is negative for a few time steps in the energy.csv file

Expected behavior
The plastic moment should increase monotonically

To Reproduce
Steps to reproduce the behavior:

1. Which version do you use? Provide branch and commit id.
   fix-707-2-nuc branch
2. Which build settings do you use? Which compiler version do you use?
   double precision, viscoelastic2, skx, order 5
3. On which machine does your problem occur? If on a cluster: Which modules are loaded?
   Frontera, modules are given in the job script
4. Provide parameter/material files.
   Ridgecrest paper setup: https://zenodo.org/record/6842773#.Y2kizL7MLUQ

Screenshots/Console output
If you suspect a problem in the numerics/physics add a screenshot of your output.

If you encounter any errors/warnings/... during execution please provide the console output.

Additional context
Add any other context about the problem here.
RC-paper-energy.csv
4973897.out.txt

[Thomas-Ulrich]

Hi,
The plastic energy calculation is here:
https://github.com/SeisSol/SeisSol/blob/master/src/ResultWriter/EnergyOutput.cpp#L349
mu and volume should be positive, so certainly pstrainCell[6 * NUMBER_OF_ALIGNED_BASIS_FUNCTIONS] gets negative.
pstrainCell[6 * NUMBER_OF_ALIGNED_BASIS_FUNCTIONS to 6 * NUMBER_OF_ALIGNED_BASIS_FUNCTIONS + NUMBER_OF_ALIGNED_BASIS_FUNCTIONS-1] stores eta decomposed in the basis of modal functions.
Eta is stored in the modal basis to enable the high-order volume output.
But eta is actually incremented in the nodal basis function:
https://github.com/SeisSol/SeisSol/blob/master/src/Kernels/Plasticity.cpp#L209:L239
I think negative Eta can arise from Gibbs effects when switching from modal to nodal and from nodal to modal.

So there would be 2 solutions:

• add a max(0, ) here https://github.com/SeisSol/SeisSol/blob/master/src/ResultWriter/EnergyOutput.cpp#L349
• (probably better) store eta in the nodal basis function, and convert to the modal basis function before the call to the
  high-order volume output.

[krenzland]

We're using a higher order quadrature than for the approximation of eta. So Gibbs effect should happen for either nodal/modal basis. Don't think there's much that we can do to avoid that.