Choosing and Verifying Contact Formulations in Ansys Mechanical Workbench for Frictional and Bonded Assemblies

Desired outcome

Select the most suitable contact formulation (Pure Penalty, Augmented Lagrange, or MPC) for a structural assembly in Ansys Mechanical Workbench and verify that the chosen setting yields acceptable penetration and convergence behavior.

Prerequisites

• Ansys Mechanical Workbench installed (version 2023R2 or later; check the Help for formulation availability).
• A simple assembly ready for contact definition – for example, two rectangular blocks that will touch under a compressive load.
• Write permission to the project folder so the solver can write solve.out and result files.
• Basic familiarity with the Mechanical tree (Contacts, Connections, Analysis Settings).

Focused procedure

1. Create the geometry and mesh
   1. In Workbench, drag a Static Structural system into the project schematic.
   2. Double‑click the Model cell to open Mechanical.
   3. Generate or import the two‑block geometry, apply a coarse mesh (e.g., 10 mm element size) to keep the example lightweight, and click Update.
2. Define a frictional contact
   1. In the Connections branch, right‑click Contacts → Contact → Frictional.
   2. Select the contacting faces of the two blocks.
   3. With the new contact selected, go to the Details view.
3. Choose a contact formulation
   1. Under Contact Properties locate the Formulation dropdown.
   2. Set it first to Pure Penalty (the default).
   3. Note the Penetration Tolerance and Contact Stiffness values; you can leave them at the program‑defined defaults for this comparison.
4. Apply loading and solve
   1. Add a fixed support to the bottom face of the lower block.
   2. Apply a downward displacement or force on the top face of the upper block (e.g., 1 mm displacement).
   3. In Analysis Settings, set Maximum Iterations to a reasonable value (e.g., 25) and enable Output Controls → Contact Information.
   4. Right‑click Solution → Solve.
5. Record penetration and convergence data
   1. After solve completes, open Solution → Contact Tool.
   2. Check the Maximum Penetration value (reported in model units).
   3. Open the solver output file (solve.out) from the project’s files folder; look for lines containing "CONVERGENCE" or "CONTACT" and note any warnings about excessive iterations or non‑convergence.
6. Repeat with alternative formulations
   1. Return to the Details view of the same contact.
   2. Change Formulation to Augmented Lagrange and repeat steps 4‑5.
   3. Change it to MPC (only available for bonded or no‑separation contact; for frictional contact the option will be greyed out – this illustrates a limitation).
   4. If you wish to test MPC, first change the contact type to Bonded (right‑click the contact → Change Type → Bonded) and then repeat the solve.

Expected checks

• Penetration: Pure Penalty typically shows a small but non‑zero maximum penetration (e.g., 0.02 mm). Augmented Lagrange should reduce this value markedly (often < 0.005 mm). MPC for a bonded pair should report zero penetration (within machine precision).
• Convergence warnings: In solve.out, Pure Penalty may converge in fewer iterations per load step but could show "Contact penetration exceeds tolerance" messages on coarse meshes. Augmented Lagrange may require more iterations but will usually report fewer penetration warnings. MPC will either solve cleanly for bonded contact or fail to solve if applied to a sliding surface (the solver will abort with an over‑constraint error).
• Contact pressure distribution: Use the Contact Pressure result to verify that pressures are smooth and physically reasonable; spiky pressures can indicate an unsuitable formulation.

Recovery options (if the chosen formulation does not work)

1. Revert the formulation: In the contact Details view, select the previous formulation from the dropdown and resolve.
2. Adjust stabilization: Increase Penetration Tolerance or decrease Contact Stiffness for Pure Penalty to reduce artificial stiffness, or increase the Augmented Lagrange penalty factor if available.
3. Refine the mesh: A finer mesh often improves convergence for Pure Penalty and reduces the need for Augmented Lagrange.
4. Change contact type: If MPC is required for a bonded region but you have sliding, consider using a bonded contact only on the truly tied surfaces and keep frictional contact elsewhere.

Limitations

• Formulation availability varies between Ansys releases; always consult the version‑specific Help (Help → Mechanical → Contact → Formulation) to confirm that MPC, Augmented Lagrange, or Pure Penalty are supported for your contact type.
• MPC formulation enforces constraints exactly and therefore cannot be used with sliding or frictional contact; applying it to such surfaces leads to over‑constraint and solver failure.
• Pure Penalty permits controlled penetration; on very coarse meshes or with large sliding distances this can produce non‑physical results even if the solver reports convergence.
• Augmented Lagrange increases solver work per iteration; for very large models this may noticeably increase solve time.

Practical verification

To confirm that the article’s guidance matches your installation, perform the two‑block test described above:

1. Run the model with Pure Penalty and record the maximum penetration (e.g., 0.018 mm) and iteration count per step (e.g., 12).
2. Switch to Augmented Lagrange and observe a reduced penetration (e.g., 0.003 mm) with a higher iteration count (e.g., 18).
3. If you change the contact to Bonded and select MPC, the penetration should read 0.0 mm and the solver should complete without penetration‑related warnings.
4. If any of these observations differ dramatically (e.g., Augmented Lagrange shows higher penetration than Pure Penalty), verify that you are using the correct Ansys release and that the contact properties have not been overridden by analysis settings.