FESADev/phases/1-structure-alignment-refactor/step6.md

Sprint Contract: MITC4 Geometry Strain Extraction

Objective

Move MITC4 geometry, director, natural-coordinate, displacement, direct covariant strain, and MITC shear-tying helpers into the Element module without changing the documented formulation.

Required Reading

• /AGENTS.md
• /PROGRESS.md
• /PLAN.md
• /docs/README.md
• /docs/HARNESS_ENGINEERING.md
• /docs/ARCHITECTURE.md
• /docs/ADR.md
• /docs/NUMERICAL_CONVENTIONS.md
• /docs/MITC4_FORMULATION.md
• /docs/VERIFICATION_PLAN.md
• /phases/1-structure-alignment-refactor/step0-architecture-map.md
• /phases/1-structure-alignment-refactor/step3.md

Scope

• Extract MITC4 natural points, tying points, director frames, geometry, integration basis, displacement derivative helpers, direct strain rows, and MITC shear tying rows.
• Keep the FESA/Abaqus S4 node order and tying-point labels unchanged.
• Keep diagnostics for invalid thickness, singular center normal, singular basis, and singular Jacobian.
• Keep drilling handling out of this step except where existing local rotation structures require it.

Allowed Files

• include/fesa/fesa.hpp
• include/fesa/Element/
• src/Element/
• include/fesa/Core/
• include/fesa/Math/
• tests/
• CMakeLists.txt
• phases/1-structure-alignment-refactor/index.json
• PLAN.md
• PROGRESS.md

Explicit Non-Goals

• Do not change MITC4 formulas, signs, tying interpolation, basis construction, or tolerances.
• Do not move stiffness integration or material matrix code in this step unless needed only to resolve declarations.
• Do not add S4R, reduced integration, hourglass control, pressure loads, or nonlinear geometry.

Tests To Write First

• Add or update include/link tests for MITC4 geometry/strain module headers.
• Preserve finite-difference, tying interpolation, geometry, basis, and diagnostic tests as characterization tests.

Reference Artifacts

• None.

Acceptance Commands

python scripts/validate_workspace.py

Evaluator Checklist

• MITC4 geometry/strain helpers live in Element.
• The implementation still matches docs/MITC4_FORMULATION.md.
• FESA tying point sign convention remains unchanged.
• No parser, assembly, or result behavior changes are mixed in.
• Existing MITC4 geometry and strain tests still pass.

Handoff Requirements

• Update this step status in phases/1-structure-alignment-refactor/index.json.
• Record moved MITC4 geometry/strain files and any remaining Element symbols still left in fesa.hpp.

Do Not

• Do not simplify or rewrite the formulation during relocation.
• Do not tune tests around floating-point drift unless a real relocation-induced bug is found and fixed.