// Complete workflow for modal/frequency analysis - extract natural frequencies and mode shapes. Use for vibration analysis and resonance avoidance.
| name | abaqus-modal-analysis |
| description | Complete workflow for modal/frequency analysis - extract natural frequencies and mode shapes. Use for vibration analysis and resonance avoidance. |
| allowed-tools | ["Read","Write","Edit","Glob","Grep","Bash(abaqus:*)","Skill"] |
Extract natural frequencies and mode shapes from a structure. Use for vibration analysis, resonance avoidance, and dynamic characterization.
Route here when user mentions:
Route elsewhere:
/abaqus-static-analysis/abaqus-dynamic-analysisBefore modal analysis:
Ask if unclear:
Route to /abaqus-geometry for part creation.
Route to /abaqus-material - density is essential.
Without density, Abaqus cannot compute the mass matrix and modal analysis will fail.
| Material | Density (tonne/mm^3) |
|---|---|
| Steel | 7.85e-9 |
| Aluminum | 2.7e-9 |
| Titanium | 4.5e-9 |
Route to /abaqus-mesh for meshing.
Mesh quality affects mode shapes - finer mesh gives more accurate high-frequency modes.
Route to /abaqus-bc to define support type.
| Configuration | Expected Modes | Use Case |
|---|---|---|
| Free-free (no BCs) | 6 rigid body modes at ~0 Hz, then elastic | Test correlation |
| Cantilever (one end fixed) | First mode is bending | Mounted component |
| Simply supported | Bending, plate modes | Bridge-like structures |
| Fixed-fixed | Higher frequencies than cantilever | Both ends constrained |
Note: Free-free analysis gives 6 modes at ~0 Hz (rigid body translation/rotation). Real elastic modes start at mode 7.
Route to /abaqus-step for FrequencyStep configuration.
Key decisions:
Route to /abaqus-job to submit, then /abaqus-odb to read frequencies from result frames.
| Parameter | Recommended | Notes |
|---|---|---|
| Eigensolver | LANCZOS | Best for most problems |
| numEigen | 10 | Start with first 10 modes |
| Normalization | DISPLACEMENT | Mode shapes max = 1 |
| Mesh size | Adequate for highest mode | Finer mesh for high frequencies |
After analysis, verify:
For cantilever beams, first mode can be verified analytically:
Compare FEA result to analytical for validation.
| Problem | Likely Cause | Solution |
|---|---|---|
| "Material has no density" | Density not defined | Add density to material |
| Negative eigenvalue | Unconstrained/unstable | Check BCs or add soft springs |
| 6 zero-frequency modes | Free-free (expected) | Real modes start at mode 7 |
| Frequencies too high/low | Unit error | Verify mm-tonne-s-N-MPa units |
| Memory error | Too many modes/elements | Reduce numEigen or coarsen mesh |
/abaqus-material - Must include density/abaqus-bc - Define modal boundary conditions/abaqus-step - FrequencyStep configuration/abaqus-odb - Extract frequencies and mode shapes/abaqus-geometry - Create geometry/abaqus-mesh - Mesh affects mode accuracyFor API syntax and code examples, see:
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