FESADev/phases/1-linear-static-mitc4-rebaseline/step7.md

Sprint Contract: MITC4 Geometry Directors

Objective

Implement or rebuild MITC4 shape functions, natural-coordinate node order, element-center director policy, local director axes, and integration-point local bases.

Required Reading

• /AGENTS.md
• /docs/MITC4_FORMULATION.md
• /docs/NUMERICAL_CONVENTIONS.md
• /docs/ADR.md

Scope

• Define N1..N4 and derivatives for FESA natural coordinates.
• Define tying points A, B, C, and D.
• Build element-center director Vn from midsurface geometry.
• Build nodal V1, V2, Vn and integration local Cartesian basis.
• Add invalid/singular element diagnostics for near-zero normal, basis, Jacobian, and thickness.

Allowed Files

• include/
• src/
• tests/
• PLAN.md
• PROGRESS.md

Explicit Non-Goals

• Do not compute stiffness.
• Do not add Abaqus *Orientation or nodal normal parsing.
• Do not add mesh quality diagnostics.

Tests To Write First

• Shape function partition-of-unity and derivative tests.
• Node order and tying-point coordinate tests.
• Flat element normal and right-handed basis tests.
• Fallback axis tests when EY x Vn is near zero.
• Invalid geometry diagnostic tests.

Reference Artifacts

• None.

Acceptance Commands

python scripts/validate_workspace.py

Evaluator Checklist

• Geometry matches docs/MITC4_FORMULATION.md.
• The old averaged-edge basis is not used as the binding formulation.
• No stress/strain or stiffness logic is slipped into this step.

Handoff Requirements

• Record MITC4 geometry readiness in PROGRESS.md.

Do Not

• Do not silently support strongly warped/nodal-director behavior beyond the documented Phase 1 policy.