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 / 2piL^2) * sqrt(EI / rhoA)

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
• Common Patterns
• Troubleshooting Guide