FESADev/phases/1-linear-static-mitc4/step11.md

Step 11: mitc4-element-baseline

Sprint Contract

Objective

Implement the baseline MITC4 element utilities and stiffness path after the formulation gate is closed.

Required Reading

• /AGENTS.md
• /PROGRESS.md
• /PLAN.md
• /docs/README.md
• /docs/HARNESS_ENGINEERING.md
• /docs/PRD.md
• /docs/ARCHITECTURE.md
• /docs/ADR.md
• /docs/NUMERICAL_CONVENTIONS.md
• /docs/MITC4_FORMULATION.md
• /phases/1-linear-static-mitc4/step10.md

Scope

• Implement shape functions, derivatives, local basis utilities, MITC shear tying per the finalized formulation, stiffness calculation, and drilling stiffness parameter path.
• Keep the element behind runtime-polymorphic element interfaces.
• Produce a 24x24 element stiffness for four-node shell elements with six DOFs per node.

Allowed Files

• include/**
• src/**
• tests/**
• docs/MITC4_FORMULATION.md only for errata discovered during implementation
• PLAN.md
• PROGRESS.md
• phases/1-linear-static-mitc4/index.json

Explicit Non-Goals

• Do not implement S4R, reduced integration, hourglass control, nonlinear kinematics, pressure loads, composites, thermal coupling, or mesh quality checks.
• Do not implement formulas that are still placeholders in docs/MITC4_FORMULATION.md.
• Do not optimize element kernels before baseline benchmark behavior is established.

Tests To Write First

• Shape function partition and nodal interpolation tests.
• Derivative and Jacobian/local basis tests.
• 24x24 dimension and symmetry tests.
• Rigid body or patch-style checks appropriate to the documented formulation.
• Drilling stiffness sensitivity tests.

Reference Artifacts

• None required for this unit-level step. Stored-reference regression is Step 14.

Acceptance Commands

python scripts/validate_workspace.py

Evaluator Checklist

• Step 10 formulation decisions are closed before code is added.
• Element interface uses runtime polymorphism as documented.
• Stiffness output dimension, DOF order, symmetry, and drilling behavior are tested.
• No S4R or nonlinear behavior was introduced.

Handoff Requirements

• Record implemented formulation surface and validation in PROGRESS.md.
• Update PLAN.md if benchmark or formulation risks remain.
• Update the matching phase index entry: use completed with a one-line summary, or blocked with blocked_reason, or error with error_message.

Verification

1. Run the acceptance command.
2. Inspect tests against the finalized MITC4 formulation checklist.

Do Not

• Do not hide formulation uncertainty in comments or TODOs inside executable code.