FESADev/phases/1-linear-static-mitc4-rebaseline/step8.md

Sprint Contract: MITC4 Covariant Strain Tying

Objective

Implement or rebuild the degenerated-continuum displacement interpolation, direct covariant strain rows, and MITC transverse shear tying interpolation.

Required Reading

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

Scope

• Implement five-DOF local displacement interpolation.
• Transform global rotations to local alpha, beta, and gamma.
• Compute direct covariant strain rows in documented internal strain order.
• Replace eps_13 and eps_23 with midside MITC tying interpolation.
• Add finite-difference tests for strain rows.

Allowed Files

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

Explicit Non-Goals

• Do not integrate stiffness.
• Do not add material transform.
• Do not compute MITC shear from Cartesian gamma_xz and gamma_yz first.

Tests To Write First

• Local rotation transform tests for alpha, beta, gamma.
• Displacement interpolation tests at midsurface and through-thickness points.
• Direct covariant strain finite-difference tests.
• Tying-row tests at A, B, C, and D.
• Gauss-point MITC shear interpolation tests proving rows come from tying rows.

Reference Artifacts

• None.

Acceptance Commands

python scripts/validate_workspace.py

Evaluator Checklist

• Strain vector ordering is documented or explicitly permuted.
• gamma does not contribute to physical continuum strain.
• FESA sign convention for A/C/B/D is tested.

Handoff Requirements

• Record strain/tying readiness in PROGRESS.md.

Do Not

• Do not reintroduce direct Gauss-point transverse shear for MITC rows.