Abaqus Boundary Conditions Skill

This skill defines boundary conditions (BCs) in Abaqus models. BCs constrain motion and prevent rigid body movement.

When to Use This Skill

Route here when user mentions:

"fixed", "encastre", "clamped", "welded"
"pinned", "hinged", "simply supported"
"roller", "sliding support"
"symmetry", "half model", "quarter model"
"constrain", "prevent movement"
"prescribed displacement", "move by X mm"
"rigid body motion error"

Route elsewhere:

Forces, pressures, gravity → /abaqus-load
Contact between parts → /abaqus-interaction
Initial temperature/stress → /abaqus-field

Key Decisions

Step 1: What Type of Support?

| User Describes | BC Type | DOFs Constrained | Physical Meaning | |----------------|---------|------------------|------------------| | "Fixed", "clamped", "welded" | Encastre | All 6 | Fully rigid connection | | "Pinned", "hinged" | DisplacementBC | U1, U2, U3 only | Rotation allowed | | "Roller", "sliding" | DisplacementBC | 1 translation | Free in-plane motion | | "Half model", "symmetric" | XsymmBC/YsymmBC/ZsymmBC | Normal + 2 rotations | Symmetry plane | | "Move it 5mm" | DisplacementBC | Specified value | Prescribed motion |

Default choice: Encastre for fixed supports (most common).

Step 2: Which Step to Apply?

| BC Purpose | Apply In | Reason | |------------|----------|--------| | Fixed support | Initial | Active before loads | | Prescribed displacement | Load step | Applied with loading | | Released BC | Later step | Use FREED to release |

Default: Apply supports in 'Initial' step.

Step 3: Rigid Body Motion Check

For 3D static analysis, constrain at least 6 DOFs total:

3 translations (X, Y, Z)
3 rotations (about X, Y, Z axes)

| Configuration | Stability | |---------------|-----------| | One face Encastre | Fully constrained | | Three pinned points (non-collinear) | Fully constrained | | One vertex + symmetry planes | May be sufficient |

"Zero pivot" error = insufficient constraints.

Step 4: Symmetry Plane Selection

| Symmetry BC | Apply When | Constrains | |-------------|------------|------------| | XsymmBC | Symmetric about YZ plane (X=const) | U1, UR2, UR3 | | YsymmBC | Symmetric about XZ plane (Y=const) | U2, UR1, UR3 | | ZsymmBC | Symmetric about XY plane (Z=const) | U3, UR1, UR2 |

Apply symmetry BC to the face AT the symmetry plane.

What to Ask User

If unclear, ask:

Where is it supported?
- "Which face/edge is fixed?"
- "Where does it mount to the frame?"
What type of support?
- "Fully fixed (welded) or can it rotate (pinned)?"
- "Free to slide in any direction?"
Is the model symmetric?
- "Can we use half symmetry to reduce model size?"
- "Is the loading also symmetric?"
Any prescribed motion?
- "Does anything move by a known amount?"
- "Is this a displacement-controlled test?"

Validation Checklist

Before running analysis:

[ ] At least one region has fixed support
[ ] All 6 rigid body modes constrained
[ ] BCs applied in correct step
[ ] Symmetry planes match actual symmetry (geometry AND loads)
[ ] No conflicting BCs on same DOF

After analysis:

[ ] Reaction forces at supports balance applied loads
[ ] No "zero pivot" or "rigid body motion" warnings
[ ] Displacements at fixed regions are zero

Common Mistakes

| Mistake | Symptom | Fix | |---------|---------|-----| | Missing BC | Zero pivot error | Add Encastre to a face | | Over-constraint | Warning in .dat file | Remove redundant BC | | BC on wrong region | Model flies away | Verify findAt coordinates | | Symmetry without symmetric load | Wrong results | Ensure loads are also symmetric | | Pinned beam (no rotation) | Unrealistic stress | Use Encastre or add rotational stiffness |

Troubleshooting

| Error | Cause | Solution | |-------|-------|----------| | "Zero pivot" | Insufficient constraints | Add more BCs | | "Negative eigenvalue" | Unstable / buckling | Check supports, may need stabilization | | "Face not found" | Wrong findAt coordinates | Use bounding box method | | "Over-constraint" | Conflicting BCs | Remove duplicate BC on same DOF |

Code Patterns

For API syntax and code examples, see: