Create uel_nlmech_pk1_Saint_Venant_Kirchhoff.for

[jpvardasca]

Summary

This pull request adds a Saint‑Venant–Kirchhoff (St. Venant) material model to the existing finite‑strain user element (UEL) framework for Abaqus/Standard. The new material routine (umatSaintVenant) replaces the previously hard‑coded Neo‑Hookean model and is selectable via the matID integer property.

Motivation

The Saint‑Venant–Kirchhoff model is a classical compressible hyperelastic model widely used for moderate strains. Including it alongside the existing Neo‑Hookean model expands the element library’s applicability, particularly for problems where a compressible, quadratic strain energy is appropriate.

Changes

• New material subroutine umatSaintVenant:

  • Computes the second Piola‑Kirchhoff stress from the Green‑Lagrange strain tensor using Lamé parameters.
  • Returns the first Piola‑Kirchhoff stress and the material tangent dP/dF for the total Lagrangian formulation.
  • Supports both 3D and plane strain analyses.

• Integration with existing UEL framework:

  • The element routine uelNLMech now calls umatSaintVenant when matID = 1.
  • The Neo‑Hookean material routine (mat_NeoHookean) remains available and is called when matID = 2.
  • The F‑bar volumetric locking treatment is fully compatible with the new material.

• Code refactoring:

  • The element routine elem_nlmech and material module hyperelastic_material from the original file are retained; the new material is added to the user_element module.
  • No changes are made to the Abaqus UEL interface, post‑processing (UVARM), or external file I/O (UEXTERNALDB).

Notes for Reviewers

• The new material routine expects three properties: Young’s modulus, Poisson’s ratio, and a locking stretch parameter (currently unused, reserved for future Arruda‑Boyce model).
• The matID must be set to 1 in the JPROPS array to activate the Saint‑Venant model.
• All post‑processed output (Cauchy stress, Eulerian strain) continues to be written to the global globalPostVars array for visualisation with overlay elements.

---

This contribution follows the existing code structure and documentation style. Feedback and suggestions are welcome.