Add Von Mises Metric for Structural Mechanics Applications

[mnabian]

PhysicsNeMo Pull Request

Description

Add von Mises stress metric for structural mechanics. This will be used in the structural mechanics recipes.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.
• The CHANGELOG.md is up to date with these changes.
• An issue is linked to this pull request.
• If I am implementing a new model or modifying any existing model, I have followed the Models Implementation Coding Standards.

Dependencies

Review Process

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[greptile-apps]

Greptile Summary

This PR adds a von_mises_from_stress_tensor function to physicsnemo/metrics/cae/von_mises.py that computes the von Mises equivalent stress from a symmetric 3×3 (or batched N×3×3) stress tensor, supporting both PyTorch and NumPy inputs. The implementation is mathematically correct, gradient-friendly (via torch.clamp instead of raw sqrt), and is paired with a thorough test suite.

• Correct formula — the distortional-energy (J₂) formula is properly implemented and verified for uniaxial, hydrostatic, and pure-shear reference cases.
• Unused parameter — _validate_stress_shape accepts an is_torch: bool argument that is never read inside the function; it is dead code and should be removed.
• Validation order inconsistency — for the numpy path, np.asarray(stress, dtype=np.float64) is called before shape validation, which may perform an unnecessary allocation for invalid inputs; the torch path validates before computing anything.
• No 2D plane-stress support — the function is limited to full 3×3 tensors; Voigt 6-vector or 2×2 plane-stress inputs are silently rejected with a ValueError. This is acceptable for the stated scope but worth noting in the docstring.
• Changelog and tests are both present and appropriate.

Important Files Changed

Filename	Overview
physicsnemo/metrics/cae/von_mises.py	New module implementing von Mises stress from 3×3 or N×3×3 stress tensors for both PyTorch and NumPy inputs. The math is correct and the gradient path is preserved. Two minor style issues: is_torch parameter in _validate_stress_shape is unused dead code, and the numpy path converts to float64 before the shape validation runs (inconsistent with the torch path).
test/metrics/test_metrics_von_mises.py	Comprehensive test suite covering uniaxial tension, hydrostatic stress, pure shear, batched inputs (both torch and numpy), invalid shapes, autograd support, and asymmetric shear symmetrization. Correctly uses the device fixture from the project-wide conftest.py.
CHANGELOG.md	One-line addition to the Unreleased section documenting the new von Mises metric module.

_{Last reviewed commit: 42c8268}

[mnabian]

/blossom-ci

[mnabian]

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