Choosing Pure Penalty or Augmented Lagrange Contact in Ansys Mechanical

A sliding assembly that will not settle

You set up a structural static analysis in Ansys Mechanical with two parts sliding against each other. The solve completes, but the contact plot shows the faces penetrating slightly, or the reaction forces look noisy when the load reverses. Tightening the mesh does not fix it, and changing the solver tolerance only slows the run. The remaining lever is usually the contact formulation.

The useful takeaway: start with Pure Penalty for simple contact, but switch to Augmented Lagrange when frictional sliding, stick-slip, or direction reversals make penetration and reaction-force accuracy more important than solve speed.

What each formulation actually does

Contact in Ansys Mechanical prevents two surfaces from occupying the same space. Pure Penalty enforces this through a contact stiffness factor: the deeper the penetration, the stiffer the resisting force. It is robust for an initial solve, but it permits small penetrations and may need manual stiffness tuning.

Augmented Lagrange adds an iterative correction on top of the penalty method. It estimates contact pressure and updates it each iteration, enforcing the contact constraint more strictly. That usually reduces penetration and improves reaction-force accuracy during frictional sliding, at the cost of extra solver iterations.

These descriptions apply to implicit static and transient structural solves. Explicit dynamics and other Ansys products use different algorithms, so do not apply this guidance outside Mechanical structural analysis without checking the release documentation.

Decision framework: when to switch

Situation	Suggested starting formulation	Why
Small-deformation static contact, little or no sliding	Pure Penalty (default)	Fewer iterations, usually adequate penetration control
Frictional sliding with repeated stick-slip	Augmented Lagrange	Stricter constraint enforcement during stick transitions
Load reversals or changing contact status	Augmented Lagrange	Better reaction-force behavior when contact opens and closes
Solver fails to converge after formulation change	Return to Pure Penalty, lower stiffness	Excessive stiffness can ill-condition the matrix

Contact stiffness is often automatic. You can adjust it under Contact Properties, but setting it arbitrarily high risks convergence failure and numerical ill-conditioning. Treat stiffness tuning as a last resort, not a default setting.

Worked example: change the contact formulation

Run this comparison in your installed Ansys Mechanical project. You need write access to the project and a mechanical license that includes contact capabilities; license entitlement varies by tier and release.

1. Open the project in Ansys Mechanical.
2. In the Tree Outline, expand Connections and double-click the contact region you want to test, for example <contact-region-name>.
3. In the Details window, open Contact Properties.
4. Locate Formulation. Select Pure Penalty, solve, and record the maximum contact penetration and the contact reaction force.
5. Change Formulation to Augmented Lagrange, solve again, and record the same outputs.
6. Compare the results using a contact penetration contour plot and a reaction force probe.

Expected check: Augmented Lagrange should show equal or lower maximum penetration and smoother reaction forces during sliding. If the Augmented Lagrange solve fails to converge or takes many more iterations, revert to Pure Penalty and inspect the stiffness factor before concluding the formulation is unsuitable.

Risk: changing formulation alters the solver’s matrix properties. A very high contact stiffness factor combined with Augmented Lagrange can produce an ill-conditioned matrix and prolonged solve times. Do not copy stiffness values from another model; verify them on your own geometry.

Limitations and how to verify

This guidance does not cover explicit dynamics, SPH, mesh-free methods, or Ansys Fluent. Ansys releases can change default contact behavior, available options, and UI paths. Before implementing, open Ansys Mechanical, insert a contact region, and inspect Contact Properties to confirm which formulation options exist in your installed release. Consult the installed Ansys help system or release notes for the contact chapter.

A practical way to check the result is a small sliding-block benchmark: build two blocks, apply a tangential load, run both formulations, and compare penetration and reaction force. Document which formulation you chose and why, because results may not replicate exactly across Ansys releases.

Closing action

Open your current sliding assembly, note the active contact formulation, and run the two-solve comparison above. If penetration is acceptable and convergence is fast, stay with Pure Penalty. If penetration or reaction-force noise is affecting your design decision, switch to Augmented Lagrange and verify convergence before accepting the results.