abaqus-bc

# 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: 1. **Where is it supported?** - "Which face/edge is fixed?" - "Where does it mount to the frame?" 2. **What type of support?** - "Fully fixed (welded) or can it rotate (pinned)?" - "Free to slide in any direction?" 3. **Is the model symmetric?** - "Can we use half symmetry to reduce model size?" - "Is the loading also symmetric?" 4. **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: - [API Quick Reference](references/api-quick-ref.md) - [Common Patterns](references/common-patterns.md) - [Troubleshooting Guide](references/troubleshooting.md)