Fiber Stretch and Stretch Rate Computations

[kko27]

Current situation

Resolves #544

Release Notes

• Added fiber stretch rate computation routine in post.cpp. Stretch rate is computed as a FD of fiber stretch
• Added integration test for stretch and stretch rates

Testing

Integration test has been added that checks if the stretch and stretch rates computed by the code match reference solutions that were obtained analytically.

Code of Conduct & Contributing Guidelines

• I agree to follow the Code of Conduct and Contributing Guidelines.

[codecov]

Codecov Report

❌ Patch coverage is 81.81818% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.02%. Comparing base (ed4c544) to head (b0cdc87).

Files with missing lines	Patch %	Lines
Code/Source/solver/post.cpp	76.78%	13 Missing ⚠️
Code/Source/solver/cep_ion.cpp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #550      +/-   ##
==========================================
+ Coverage   68.80%   69.02%   +0.21%     
==========================================
  Files         181      181              
  Lines       34157    34128      -29     
  Branches     5903     5904       +1     
==========================================
+ Hits        23503    23557      +54     
+ Misses      10517    10434      -83     
  Partials      137      137              

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[aabrown100-git]

Nice work, made a few suggestions

[michelebucelli]

Thank you @kko27! I have left a few suggestions and comments.

[javijv4]

Hey @kko27, thanks for working on this. Besides the comments I left, I asked Codex to run a review, and it found this issue:

In cep_ion.cpp (line 160), post::fib_stretch(simulation, iEq, ...) is called with iEq for the CEP equation. But compute_fib_stretch() (line 745) uses com_mod.eq[iEq].s at post.cpp (line 757) to choose which rows of the global displacement state feed F at post.cpp (line 791). In a coupled CEP+struct run, the CEP equation’s s is not the structural displacement offset, so the stretch-activated current path can be driven by unrelated data.

I think it's right. You are passing the EP index equation, not the mechanics, to fib_stretch, so that's going to fail. I don't think this PR addresses the electromechanics coupling, so this is a future issue, but it might be worth commenting out (or deleting?) the fib_stretch call in cep_ion.cpp until we have tested it?

[Copilot]

Pull request overview

Adds solver support and a new integration case to compute and validate fiber stretch and fiber stretch rate outputs needed for electromechanics post-processing (resolves #544).

Changes:

• Introduces Fiber_stretch and Fiber_stretch_rate outputs end-to-end (enums, output properties, VTU writer, post-processing routines).
• Refactors deformation gradient computation into a shared helper and implements stretch rate via backward finite difference.
• Adds a new structural integration test case (uniaxial_block_stretch) with reference results for both fields.

Reviewed changes

Copilot reviewed 23 out of 23 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
Code/Source/solver/post.cpp	Adds fiber stretch + stretch-rate post routines; refactors deformation gradient usage.
Code/Source/solver/post.h	Declares new/renamed post-processing APIs (fib_stretch, fib_stretch_rate).
Code/Source/solver/vtk_xml.cpp	Writes new scalar nodal outputs to VTU (Fiber_stretch, Fiber_stretch_rate).
Code/Source/solver/consts.h	Extends OutputNameType with new output groups and output names.
Code/Source/solver/set_output_props.h	Registers output props (group, length, VTK name strings) for new outputs.
Code/Source/solver/set_equation_props.h	Adds new outputs to struct/ustruct default output lists and updates nDOP.
Code/Source/solver/cep_ion.cpp	Updates call site to renamed post::fib_stretch().
tests/conftest.py	Adds absolute tolerances for the new output fields in reference comparisons.
tests/test_struct.py	Adds integration test entry for the new uniaxial block stretch case.
tests/cases/struct/uniaxial_block_stretch/*	New integration test case assets (solver input, mesh, load, reference VTU, docs/media).

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[kko27]

Hey @kko27, thanks for working on this. Besides the comments I left, I asked Codex to run a review, and it found this issue:

In cep_ion.cpp (line 160), post::fib_stretch(simulation, iEq, ...) is called with iEq for the CEP equation. But compute_fib_stretch() (line 745) uses com_mod.eq[iEq].s at post.cpp (line 757) to choose which rows of the global displacement state feed F at post.cpp (line 791). In a coupled CEP+struct run, the CEP equation’s s is not the structural displacement offset, so the stretch-activated current path can be driven by unrelated data.

I think it's right. You are passing the EP index equation, not the mechanics, to fib_stretch, so that's going to fail. I don't think this PR addresses the electromechanics coupling, so this is a future issue, but it might be worth commenting out (or deleting?) the fib_stretch call in cep_ion.cpp until we have tested it?

Oh dear, this will definitely become a problem for coupling. We don't need to address this in this issue as right now users cannot enable electromechanics coupling, but I will add this note to the main issue #523

[kko27]

@michelebucelli and @javijv4 I think this is ready for final review and approval

[javijv4]

Looks good to me!