Align USERMAT parameters with ANSYS Neo-Hookean D1

[DexterQiu]

Motivation

• Ensure the USERMAT implementation and documentation use the same incompressibility parameter naming as ANSYS Neo‑Hookean (D1) so property mapping and comments match user expectations.
• Clarify the strain energy and stress expressions to reference D1 instead of the previous generic d/DPEN naming.
• Keep the README APDL snippet consistent with the Fortran implementation so example runs use the correct property order.

Description

• Rename internal/procedure parameters from DPEN/d to D1 and update all occurrences in usermat_neohookean.f90 (variables, comments, and formulae) to reflect ANSYS D1 semantics.
• Update calls and the NEOHOOK_S2PK subroutine signature to accept D1 and use (1/D1) in the compressible penalty formulation W = (mu/2)*(I1-3) + (1/D1)*(J-1)^2.
• Adjust effective penalty ramping logic to compute D1_EFF and propagate it through stress/energy/tangent computations.
• Update README.md APDL snippet to list PROPS as mu, D1, ramp_time so the example matches the code.

Testing

• No automated tests or CI jobs were executed for these changes.
• Static inspection and local edits were performed to ensure consistent renaming across usermat_neohookean.f90 and README.md.

---

Codex Task

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: ea9a84be44

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

[chatgpt-codex-connector]

Guard against D1=0 before dividing

If a user supplies D1=0 (a common ANSYS convention for incompressible behavior), the expression (2.0D0/D1) will divide by zero and propagate Inf/NaN into stresses and tangents, typically causing solver failure. Consider explicitly rejecting D1<=0 with a clear error or replacing it with a small positive floor before computing stresses/energy.

Useful? React with 👍 / 👎.