| name | calculix-sizing-optimization |
| description | Workflow skill for two-stage sizing/parameter optimization on a CalculiX shell or beam deck via the optimize_structure tool — Latin Hypercube sweep plus coordinate descent to minimize mass subject to stress/displacement or natural-frequency constraints by tuning scalar section/material/load cards. Use when an agent must lighten a CalculiX shell or beam model while keeping stress and deflection within limits, or must thin a deck until its first modes clear a resonance floor (freq_1_hz constraint on a *FREQUENCY deck). |
CalculiX Sizing Optimization
Two-stage sizing/parameter optimization: minimize mass subject to stress,
displacement, or natural-frequency constraints by editing scalar design
variables in place (shell thickness, beam section, material E/nu/density, load
magnitude). The mesh and geometry never change — only scalar cards.
This is sizing optimization, not topology optimization. It thins sections;
it does not redistribute material in space.
When to Use
Use when an agent must lighten a CalculiX shell or beam model while keeping
von Mises stress and displacement within limits (static deck), or must lighten
it while keeping a natural frequency above a resonance floor (modal deck).
Driven by the optimize_structure_tool MCP tool.
Do NOT use for:
- Solid (C3D8 / C3D8R) models. Solids expose no scalar geometry card — their
mass is set by node-defined volume x density, so there is no thickness to
thin. Material/load variables on a solid are degenerate for mass minimization
(density changes mass but not stiffness; E changes stiffness but not mass).
Solid lightweighting needs shape or topology optimization, which is a
different problem and is not covered here.
- Topology optimization (material distribution over a fixed mesh) — separate,
future work.
Workflow
parse_inp / list_design_vars_tool — confirm the deck and find the
shell.<elset>.thickness (or beam section) var_id and its current value.- Choose bounds
{var_id: [lower, upper]} to bracket the search. Mass falls
monotonically with shell/beam thickness.
optimize_structure_tool — run the two-stage loop (LHS sweep, then
coordinate descent). Each evaluation is a real ccx solve, so set max_solves
to bound wall time.- Inspect the result:
best (vars, mass_kg, stress_vm, disp, feasible,
mass_reduction_pct), converged / termination_reason, and history.
- Optional:
export_results_tool on the persisted <stem>.optimized.inp to
render the optimized design in the viewer.
Rules
- Frame results as sizing/parameter optimization (section sizing), never
topology.
- Defaults: minimize mass s.t. max von Mises < 250 MPa and max displacement <
1.5 mm; pass
objective / constraints to override.
- Match the constraint set to the deck: a
*STATIC deck reports
max_stress_vm / max_disp; a *FREQUENCY deck reports freq_<N>_hz
(mode N in Hz, from the .dat eigenvalue table) and nothing else. Mixing a
stress constraint into a modal optimization makes every point infeasible —
the run warns about missing metrics rather than failing opaquely.
- Avoid-resonance runs: on a modal deck pass e.g.
constraints=[{"metric": "freq_1_hz", "op": ">", "value": 300.0}]; thinning
stops where mode 1 sits just above the floor. Constraint metric names are
validated (mass, max_stress_vm, max_disp, freq_<N>_hz).
- The acceptance rule assumes shell/beam thickness (mass-monotone). Material E
and load magnitude are exposed as variables but are not validated for
mass-minimization — prefer section thickness.
- Units follow the
.inp (commonly mm-t-s-MPa); mass_kg is reported in kg.
- A
converged=False result is not a failure: best is the lightest feasible
point found, and bound_limited tells whether it already sits at the box
optimum (widen the bounds to do better).
Example
MCP/CalculiX/examples/bracket.inp is a public S4 shell bracket (steel plate,
clamped edge, transverse tip load). Starting from thickness 8 mm with bounds
{"shell.PLATE.thickness": [2.0, 8.0]} and n_lhs=8, the optimizer converges
to ~4.1 mm — about -48% mass — while keeping stress < 250 MPa and
displacement < 1.5 mm.
MCP/CalculiX/examples/plate_modal.inp is the avoid-resonance counterpart: a
public S4 shell cantilever plate with a 5-mode *FREQUENCY step. With
constraints=[{"metric": "freq_1_hz", "op": ">", "value": 30.0}] the
optimizer thins from 4 mm to ~3.23 mm — -19% mass, f1 = 30.4 Hz — matching
the Euler-Bernoulli hand calc t* = 30 / 9.29 ≈ 3.23 mm for L = 300 steel
(f1 ≈ (1.8751²/2π)(t/L²)√(E/12ρ)).
