How to Set Up and Verify Contact in an FE Model

1. The Engineering Task

Define contact between two or more surfaces such that the model correctly prevents penetration, transfers normal pressure and frictional shear, and converges reliably.

2. When to Use This Method

Contact is required wherever two bodies can press against each other, separate, or slide. Common applications include bolted joints, pin-lug connections, interference fits and structural gaps. Contact is not needed where surfaces are permanently bonded — use tie constraints instead.

3. What You Need Before Starting

• Identification of the master and slave surfaces (typically the coarser mesh is the master)
• The coefficient of friction appropriate to the material pair and surface condition
• Knowledge of whether the contact is likely to open, close or slide during the analysis
• The expected contact pressure magnitude — this affects the contact stiffness scaling

4. Step-by-Step Method

1. Identify all surface pairs that may come into contact during the analysis
2. Assign master and slave surfaces. The slave surface should have the finer mesh for accurate contact pressure distribution
3. Define the contact interaction: normal behaviour (hard contact or penalty stiffness) and tangential behaviour (Coulomb friction)
4. Set the coefficient of friction based on test data or published values for the material pair and surface finish
5. Choose the contact discretisation: surface-to-surface is more accurate but more expensive; node-to-surface is faster but can produce noisier contact pressure
6. For initial contact problems (interference fits, preloaded joints), define the initial clearance or interference in the contact property
7. Run the analysis and check convergence. If the analysis struggles to converge, adjust the contact stiffness or use automatic stabilisation

5. What to Check

• Contact status: are the contacting surfaces in contact where expected and open where expected?
• Contact pressure: is the pressure distribution smooth and physically plausible? Spike pressures at element boundaries indicate the slave mesh is too coarse
• Penetration: check that penetration of the slave into the master surface is small relative to the element size
• Sliding displacement: is the sliding distance consistent with the friction coefficient and applied load?
• Convergence: examine the contact force residuals — large residuals indicate contact chattering

contact-interface

If contact pressure shows a checkerboard pattern, the mesh on the slave surface is too coarse relative to the master. Refine the slave mesh or switch to a finer contact discretisation.

6. How to Interpret the Result

A correct contact solution shows smooth contact pressure distribution over the contacting region, no significant penetration, and converged force residuals. The contact status should be physically sensible: regions that should be open are open, and regions that should be in contact show positive pressure. If bolt preload is included, the contact pressure under the bolt head should be consistent with the preload magnitude.

7. Common Mistakes

• Using hard contact without stabilisation — can cause convergence failure when contact opens or closes
• Setting the friction coefficient to zero "to be conservative" — this changes the load path and may not be conservative for all failure modes
• Failing to refine the mesh at the contact interface — produces noisy pressure distribution
• Not checking for initial penetration or gap — can introduce spurious contact forces at the start of the analysis
• Ignoring contact in a "linear" analysis where gaps may open under load — the result is not physically correct

8. Further Reading

See the Joints & Interfaces Knowledge category for contact mechanics theory. See How to Model Bolts and Fasteners for bolted contact and preload application.