What is Large Deflection in FEA and When Should You Turn It On?

Introduction

In structural analysis, the Large Deflection option (also called geometric nonlinearity) determines whether the solver accounts for geometry changes during loading. Many analysts overlook this setting, but enabling or disabling it can significantly affect accuracy.

Most commercial FEA software keep Large Deflection OFF by default. This makes sense because many models involve small deformations where geometric effects are negligible, and turning it ON unnecessarily increases computation time. However, in certain scenarios, leaving it OFF can lead to major inaccuracies.

This article explains what Large Deflection does, why it matters, and when to enable it—including quantitative thresholds, interaction with other nonlinearities, and solver-specific considerations.

What Does Large Deflection Do?

When Large Deflection is enabled, the solver updates the global stiffness matrix of the model as the geometry deforms. The stiffness matrix represents how the structure resists loads, and it depends on the current shape.

If a structure bends or rotates significantly, its orientation changes, altering the direction of internal forces and stiffness characteristics. Ignoring these effects can lead to incorrect predictions.

Linear vs. Large Deflection Behavior

Linear Geometry Assumption (Default in Most Software)

• The stiffness matrix is assembled once using the original geometry.
• Displacements are assumed small enough that geometry changes do not significantly affect stiffness or force directions.
• External loads can change magnitude, but their direction remains fixed relative to the original configuration (unless defined as follower loads).

With Large Deflection Enabled

• The stiffness matrix is updated at each iteration to account for geometry changes.
• Internal forces and load paths adjust as the structure deforms.
• Essential for problems involving:
  • Large rotations (e.g., bending, twisting)
  • Follower forces (e.g., pressure tracking surface motion)
  • Geometric instability (e.g., buckling, snap-through)

Example: The Fishing Rod Analogy

Imagine a fishing rod held horizontally with a weight at the tip:

1. Initially, the rod resists mainly through bending.
2. As it bends downward, part of the load acts axially, increasing stiffness.
3. A linear analysis ignores this stiffening effect, leading to overpredicted displacements.

Large Deflection ensures these geometric effects are captured correctly.

What Happens If You Don’t Turn It On?

If deformations are very small, leaving it OFF is fine. However, for large deformations or rotations, problems include:

• Inaccurate displacements & stresses (stiffness matrix never updates).
• Contact surfaces behaving incorrectly (outdated geometry references).
• Follower loads (e.g., pressure) not tracking surface motion.
• Inability to capture buckling or snap-through instabilities.

When Should You Enable Large Deflection?

Enable Large Deflection when geometry changes significantly influence stiffness and force distribution.

1. Large Rotations or Displacements

• Example: A cantilever beam tip deflecting > 5–10% of its length or rotating > 5°.
• Rule of thumb: If displacements exceed ~10% of the smallest structural dimension, enable it.

2. Elastic-Plastic Analyses

• Always enable when plasticity is present—material and geometric nonlinearities interact.
• Large strains (e.g., rubber, metals after yielding) require both Large Deflection and proper material models.

3. Instability Problems

• Essential for post-buckling or collapse analysis (linear buckling eigenvalue analysis does not require it).

4. Contact with Major Sliding or Separation

• Needed for accurate contact tracking (e.g., seals, gaskets, snap-fit assemblies).

5. Follower Forces

• Loads like pressure or centrifugal forces must follow surface orientation.

6. Assemblies with Joints

• If joints (revolute, cylindrical, etc.) rotate significantly, Large Deflection ensures proper force transfer. This is illustrated by the following common warning message in Ansys:
  
  “Joints are being used in the current analysis with Large Deflection turned off. Thus, only linearized joint behaviour will be considered. If a finite rotation and large deflection effects are to be considered, please turn on Large Deflection.“

Note: Some industry standards (e.g., offshore, pressure vessel, or seismic codes) may explicitly mandate geometric nonlinearity—always verify compliance.

Consider the Analysis Objective

• Preliminary screening or mesh studies? Leave it OFF if deformations are small.
• Optimization? Enable if geometric effects impact design variables.
• Final validation or safety-critical checks? Always enable if large deformations are possible.

Performance Impact

• Enabling Large Deflection increases solution time (stiffness matrix updates every iteration).
• May require smaller load increments for convergence.
• No benefit in small-strain problems—use only when needed.

Common Scenarios Table

Scenario	Large Deflection?	Notes
Plate bending (small displacement)	❌ No	Linear assumption valid
Cantilever beam (large tip rotation)	✅ Yes	>5° rotation or >10% length deflection
Cable sagging under self-weight	✅ Yes	Geometric nonlinearity dominates
Elastic-plastic analysis	✅ Yes	Required for accurate plasticity
Joints in rotating mechanisms	✅ Yes	Ensures correct force transfer
Linear buckling (eigenvalue)	❌ No	Post-buckling requires it

Software-Specific Notes

• ANSYS: “Large Deflection” ON enables geometric nonlinearity.
• Abaqus: “NLGEOM” (Nonlinear GEOMetry) accomplishes the same.
• COMSOL: “Geometric Nonlinearity” checkbox in study settings.
• SolidWorks Simulation: “Large Displacement” option.

Final Thoughts

Large Deflection is usually OFF by default because many problems involve small deformations. However, in cases with:

• Large rotations/displacements (>10% length or >5° rotation),
• Plasticity,
• Follower forces,
• Contacts with large sliding,
• Instability (buckling, collapse),

Enabling it could be critical for accuracy. Always assess whether geometric nonlinearity impacts your results before deciding.

Key Takeaways

1. Use Large Deflection when deformations are “large” (deflection >10% of part size or rotations >5°).
2. Always enable it for plasticity, follower forces, or buckling/post-buckling.
3. Check solver-specific settings (e.g., NLGEOM in Abaqus).
4. Balance accuracy vs. computational cost—turn it off for small-strain linear problems.

Further Readings:

Small Sliding in ANSYS