Sometimes during ANSYS structural simulations, messages like the one “Too Much Penetration at Contact Points” may pop up, saying something about “contact penetration.” It can confuse some people, so let’s talk about it a bit.
1. This message isn’t an error message or a warning message.
ANSYS solver displays a lot of messages during the solution process, and some of them are labeled as warnings (which means there might be a problem) or errors (which means the solution failed). But the penetration message is just there to give you some information.
That being said, context is important. Sometimes, the solver will output messages that aren’t a big deal on their own, but when taken together with other messages, they can be really helpful. For example, if the solver is having a hard time converging to a solution, the penetration message might be a clue as to why that’s happening.
So, to sum up: if you see a penetration message during your ANSYS simulation, don’t panic. It’s just there to give you some information, and it might be useful in figuring out what’s going on with your model.
2. What is contact penetration?
Contact penetration is a concept used in ANSYS. ANSYS has two categories of contacts: Normal Lagrange and Penalty Based. The way the contact interaction is treated mathematically differs between these two methods.
Figure 1: Contact Penetration
Normal Lagrange method is a step function where a contact point is either open or closed. However, this type of contact can sometimes result in chattering, where the contact cycles between open and closed states, and the model does not converge to a solution.
To address chattering, contact penetration is introduced. Nodes on the interfacing bodies are allowed to interpenetrate, but penetration is kept to a predefined minimum value. An artificial spring is used to “pull” the red surface back and minimize penetration. A low enough value of penetration is required for a converged solution.
Figure 2: Contact Faces
3. What does this message mean?
If ANSYS reports that the amount of penetration at a contact pair is more than a predefined value, it means the solver is in the process of achieving the desirable level of penetration. This message may disappear after a few iterations. However, if this message continues to appear and there is difficulty in converging to a solution, the source of instability within the model needs to be identified and mitigated.
4. Using this message to debug an unstable model
Figure 3 shows an example of this, where we can see a warning message before the penetration message.
Figure 3: Contact Warning Messages
The warning message tells us that the contact status has changed suddenly and gives us the real ID of the contact (593). This is helpful information because it tells us that contact 593 is going through rapid changes and experiencing too much penetration.
To solve this problem, we need to identify which contact in the model is associated with the real ID 593.One way to do this in workbench is by inserting a contact tool and identifying the contact with real ID 593.
Figure 4 shows the initial contact information as processed by the contact tool. We can right click on the contact definition with “Real Constant” 593.
Figure 4: Contact Tool Initial Infromation
Now that the contact has been identified we can consider the following aspects:
- Is the geometry modelled correctly?
- Are the loads and boundary conditions correctly defined?
- Is the mesh sufficiently fine and of sufficient quality?
- Are the contact surfaces correctly defined?
- Do we need to apply the load more slowly (add more sub-steps)?
- Do we need to consider the effects of material non-linearities and element distortion?
- Do we need to consider changing the contact settings (contact type, interface treatment, contact stiffness, damping force etc.)
It’s essential to pay attention to these points to determine the source of the instability and make the necessary changes to achieve stable behavior.
Pro Tip: Based on my experience, when dealing with excessive penetration and non-convergence issues, we should pay particular attention to mesh quality and the normal stiffness factor. The default normal stiffness factor is 10.0 for bonded contacts and 1.00 for non-bonded contacts. Lowering the normal stiffness factor from 1.00 to 0.20 for non-bonded contacts can help greatly with excessive penetrations and non-convergence issues.