abaqus-modal-analysis

# Abaqus Modal Analysis Skill Extract natural frequencies and mode shapes from a structure. Use for vibration analysis, resonance avoidance, and dynamic characterization. ## When to Use This Skill **Route here when user mentions:** - "Natural frequency", "modal analysis", "vibration" - "Resonance", "mode shapes", "eigenvalue" - "How will it vibrate?", "avoid resonance at X Hz" - "First mode frequency", "natural frequency of beam/plate" **Route elsewhere:** - Forced vibration response → use transient dynamic - Frequency response function → use steady-state dynamics - Static stress/deflection → `/abaqus-static-analysis` - Impact/crash → `/abaqus-dynamic-analysis` ## Prerequisites Before modal analysis: 1. Geometry and mesh ready 2. Material MUST have density defined (required for mass matrix) 3. Boundary conditions define the modal boundary 4. NO loads needed for eigenvalue extraction ## Workflow: Modal Analysis ### Step 1: Understand User's Goal Ask if unclear: - **How many modes?** First few (5-10) or all in frequency range? - **Boundary conditions?** Fixed, pinned, free-free? - **Frequency range of interest?** Motor at 60 Hz, etc.? - **What geometry?** Beam, plate, bracket, assembly? ### Step 2: Create Geometry Route to `/abaqus-geometry` for part creation. ### Step 3: Define Material WITH DENSITY 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 | ### Step 4: Create Mesh Route to `/abaqus-mesh` for meshing. Mesh quality affects mode shapes - finer mesh gives more accurate high-frequency modes. ### Step 5: Apply Boundary Conditions 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. ### Step 6: Create Frequency Step Route to `/abaqus-step` for FrequencyStep configuration. Key decisions: - **Fixed count:** Extract exactly N modes (numEigen=10) - **Frequency range:** All modes between min and max Hz - **Shift-invert:** Modes near target frequency (for high-frequency focus) ### Step 7: Run and Extract Route to `/abaqus-job` to submit, then `/abaqus-odb` to read frequencies from result frames. ## Key Parameters | 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 | ## Validation Checklist After analysis, verify: - [ ] Density defined in material - [ ] BCs match intended support condition - [ ] No loads applied (eigenvalue extraction ignores loads) - [ ] Mesh adequate for highest mode of interest - [ ] Frequencies reasonable for geometry/material - [ ] Free-free: confirm 6 modes near 0 Hz ## Analytical Comparison (Simple Geometries) For cantilever beams, first mode can be verified analytically: - f1 ~ (1.875^2 / 2*pi*L^2) * sqrt(E*I / rho*A) Compare FEA result to analytical for validation. ## Troubleshooting | 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 | ## Related Skills - `/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 accuracy ## Code Patterns For API syntax and code examples, see: - [API Quick Reference](references/api-quick-ref.md) - [Common Patterns](references/common-patterns.md) - [Troubleshooting Guide](references/troubleshooting.md)