| name | calculix-modal-analysis |
| description | Workflow skill for CalculiX modal analysis (*FREQUENCY steps) — natural frequencies and mode shapes via the CalculiX MCP. read_results returns the frequency table; export_results with mode=N renders that mode shape in the browser viewer. Use when an agent must extract natural frequencies, check resonance margins, or visualize mode shapes on an open-source solver. |
CalculiX Modal Analysis
Free-vibration eigenanalysis on a CalculiX deck: a *FREQUENCY step requests N
eigenpairs; ccx prints the eigenvalue table and per-mode eigenvectors to the
.dat, so both frequencies and mode shapes parse from text (no .frd needed).
When to Use
Use when an agent must extract natural frequencies, check resonance/vibration
margins, or visualize mode shapes on a CalculiX model. Driven by the same MCP
tools as static runs (run_solver → read_results → export_results).
Not for: static stress/deflection (use calculix-fem), transient/dynamic
response (not yet supported), or sizing optimization (calculix-sizing-optimization).
Workflow
- Confirm the deck has
*DENSITY under its *MATERIAL — frequencies need
mass; without density ccx fails the eigenvalue solve.
- Confirm the step is
*FREQUENCY with the wanted mode count on its data
line, and *NODE PRINT, NSET=<all> / U so eigenvectors reach the .dat.
No load is applied (free vibration); keep the *BOUNDARY clamp set.
run_solver_tool — submit the deck. Note: a *FREQUENCY run writes a
header-only .sta (no increments in an eigenvalue solve); success
accepts a non-empty .dat instead, so this is normal, not a failure.read_results_tool — returns frequencies as
[{mode, eigenvalue, freq_rad_s, freq_hz}] and n_modes (a doubly
symmetric section gives degenerate pairs — f1 = f2 — which is expected).export_results_tool with mode=N — writes result_mesh.json holding
that mode's eigenvector as a stress-free displacement field; the viewer
renders the mode shape with its usual auto-scaled deformation.
Rules
- Units follow the
.inp (commonly mm-t-s-MPa → frequencies in Hz).
- Eigenvectors are mass-normalized; their magnitude carries no physical
displacement meaning — only the shape does. The viewer auto-scales.
- Fully-integrated C3D8 hexes shear-lock in bending: ccx frequencies run
~5-10% ABOVE the Euler-Bernoulli hand calc. For tight margins, refine the
mesh through the thickness (or switch element type) and report the gap.
- Hand-calc check for a clamped-free bar: f_n = (beta_n^2 / 2pi) *
sqrt(E I / (rho A L^4)), beta_1 = 1.8751.
Example
MCP/CalculiX/examples/cantilever_modal.inp is the public cantilever benchmark
with a 5-mode *FREQUENCY step. ccx gives f1 = f2 ~ 502 Hz (degenerate bending
pair on the square section) vs the Euler-Bernoulli hand calc ~ 464 Hz (+8%,
C3D8 shear locking). Mode 1 exports straight into the viewer as the classic
half-sine bend.
