calculix-modal-analysis

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

1. Confirm the deck has *DENSITY under its *MATERIAL — frequencies need mass; without density ccx fails the eigenvalue solve.
2. 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.
3. 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.
4. 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).
5. 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.