ashfordeou/aero-agent-skills/calculix-linear

CalculiX Linear Static FEA (structures/fem/calculix-linear)

Use when the task is linear static finite element analysis of aircraft structure with CalculiX (ccx): stress post-processing, margin of safety checks against material allowables, and unit discipline across the analysis chain.

Domain quick reference

• Margin of safety: MS = allowable / actual - 1.0; a negative MS

means the structure fails at the applied load.

• Von Mises equivalent stress: sqrt(0.5*((s1-s2)^2 + (s2-s3)^2
• (s3-s1)^2)); compare it against allowables for ductile metals.
• Unit discipline: allowables and FEA stresses must be compared in

the same unit. Conversion factors to Pa: kPa 1e3, MPa 1e6, GPa 1e9, psi 6894.757, ksi 6894757.0.

• FAR-25 structure requirements (25.301-25.307 loads and

structural proof) set the certification context for the stress checks; this skill computes the checks, not the loads.

Workflow

1. Confirm the analysis is linear static (small displacements,

linear materials) and record the unit convention of the ccx run.

1. Extract stresses at the element or node of interest from the

ccx output; record the unit.

1. Convert the FEA stress and the allowable to a common unit with

stress_to_pa / mos_units_discipline.

1. Compute margin of safety with margin_of_safety and classify

with mos_status; negative margin = failed structure.

1. Check equivalent stress with von_mises where a scalar

comparison is needed.

1. Confirm the deterministic checks with the contract test

scripts/test_calculix.py.

Pitfalls

• Comparing allowables and FEA stresses in different units

(silent unit errors); always convert through Pa.

• Treating a negative margin of safety as acceptable; it means the

applied stress exceeds the allowable.

• Using von Mises for brittle or directionally dependent

materials without justification.

• Applying linear-static checks to nonlinear or buckling-dominated

responses.

Behavior contract (gate 3)

The margin, unit, and von Mises logic is exercised by the gate 3 contract test: scripts/test_calculix.py against scripts/calculix_logic.py (stdlib unittest, offline). Run: python3 scripts/test_calculix.py

Compliance

• Standards referenced, not reproduced: FAR-25 is US government

work (public domain); summary-only per standards-map.yaml and brief 06.

• compliance: STANDARDS-REF, gated: false.