Introduction
When conducting a FEA with contact nonlinearities, often convergence difficulties occur due to said contacts. Fortunately, Ansys Mechanical has a set of tools that help diagnose the cause of divergence. In this article, we will discuss how to utilize Contact Trackers in the Solution Information branch to diagnose convergence issues and to determine if the contact leads to a correct solution.
Example FE Model
The example model is an axisymmetric piston-cylinder assembly, sealed with an O-ring; it comes from the Ansys Mechanical Advanced Connections course from the Ansys Learning Hub (ALH). Here, the assembly of the system is simulated in two steps:
- Install the O-ring into the piston seal groove.
- Install the cylinder onto the sealed piston by translating the cylinder along the piston's axis.
Contact Definition
The contacts between the O-ring and the piston and between the O-ring and the cylinder are frictional with coefficient of friction = 0.2. The following images show the contact geometry and definition for both contacts:
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Analysis Settings
The Analysis consists of two steps: 1) resolve the interference between the O-ring and the piston, and 2) displace the cylinder vertically to complete assembly. The required Analysis Settings required are as follows:
Additionally, Weak Springs = Off, Large Deflection = On, and Newton-Raphson Option = Unsymmetric (in Nonlinear Controls).
Contact Trackers
Contact Trackers provide a method to gain information about contact behavior during the solution process. Contact Trackers display time series graphs of maxima or minima of key metrics; they are not contour plots, so they are light weight.
Using two PyMechanical scripts, Contact Trackers for seven different Definitions, grouped by contact, and Contact Trackers for Force Convergence alone are rapidly created. Here is the resulting Solution Information branch layout:
Solution Information
Base Case
Using Program Controlled settings for contact Normal Stiffness and Update Stiffness, we achieve the following Force Convergence behavior:
Here we see that several bisections occur, so we would like to understand their source. Using a Force Convergence contact tracker, we are able to see if any particular contact contributes to bisections or poor convergence behavior. Here are plots of the Force Convergence Contact Tracker for each contact where we see that each contact contributes to convergence challenges at different points in time:
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Finally, to complete the exposition of the base case, we plot the (maximum) Penetration for each contact, where we take note of the resulting value at the end of each time step:
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Softer Contact
Now, we will change the Normal Stiffness and Update Stiffness to Factor = 0.1 and Each Iteration, Aggresive, respectively. After solving, the Force Convergence curve shows that fewer bisections occur, but 80 more iterations were required:
Force Convergence Contact Trackers show very clean convergence behavior due to the contacts:
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However, reviewing the Penetration values, we see that there is no free lunch: as we soften the contacts, we more easily achieve convergence at the cost of more contact penetration:
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Stiffer Contacts
As a final test, we will now increase the Normal Stiffness to have Factor = 10. In this case, we have divergence with many more bisections as seen by the Force Convergence behavior:
Reviewing the Force Convergence Contact Trackers, we see that the O-ring to Cylinder contact is the culprit, and we likely may be able to maintain the stiffer contact settings for the O-ring to Piston contact:
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After returning Normal Stiffness and Update Stiffness to Program Controlled for the O-ring to Cylinder contact, we obtain converge again:
Conclusions
In this article, we showed how the Contact Trackers are useful in diagnosing contact related convergence problems. Ideally, we would like to solve the model with the stiffest contacts that allow for convergence in an effort to minimize contact penetration. We showed that there is a trade-off between contact softening and convergence behavior: the softer the contact, the more penetration; and sometimes it is necessary to soften contacts to achieve convergence. In summary. using contact stiffness settings and Contact Tracker monitoring, one can iterate the model in order to achieve an accurate solution that has desired convergence behavior.
Going Further
- Using the procedure outlined here, perform a contact convergence study to see how stiff the contacts need to be to obtain a converged solution.
- Refine the mesh to see how mesh refinement and contacts work in tandem toward achieving a converged solution.
Downloadable Resources
Ansys 2025 R1 SP1 Workbench model archive and python scripts
