We regularly need to model contact between parts within FEA models. After all, not many of our simulations will include only one component, and as we begin to increase the complexity of our jobs, we tend to prefer contact over traditional ties and glue. On occasion, it is simply not possible to obtain static equilibrium in a static simulation without some help. Of course, we’re talking about rigid body motion again.
If we’re modeling contact between components that begin life separated or are difficult to position exactly due to the nature of our problem, then rigid body motion can become a real pain. That’s because where there is rigid body motion, there is never static equilibrium.
Fortunately, Abaqus and many other commercial FEA codes provide help with this annoying problem – contact stabilization. In this post, we’ll explain, in real terms, what contact stabilization is and when we should use it. We’ll also show you exactly when you shouldn’t use it, or, rather, when you shouldn’t trust your results.
What Is Contact Stabilization?
If we go to the Abaqus manual, we’ll find contact stabilization defined quite succinctly:
“Abaqus offers contact stabilization to help automatically control rigid body motion in static problems before contact closure and friction restrain such motion”
By applying it, we’re effectively applying forces that react against rigid body motion to allow Abaqus to find a solution to static equilibrium where there really isn’t one. Think of it like putting our contact pair in a very viscous fluid like honey or syrup that restrict the relative motion of the contact pair until they meet.
The damping forces are automatically calculated based on the clearance between the contact pair and are only activated once the distance between the contacting surfaces is less than a characteristic surface dimension. As the step progresses and (ideally) the contact begins to restrict motion by itself – by way of pressure and friction - the contact stabilization is ramped down, linearly, to zero. As you might imagine, this means that the results at the end of the step are (usually) valid. However, care must be taken not to blindly trust results that have been generated using contact stabilization.
When You Should Use It
There is no doubting that, sometimes, you just have to use contact stabilization to get a model to converge, but most FEA software developers would tell you that this really should be considered a last resort. It is highly advisable that you first try to stabilize rigid body motion through robust modeling techniques such as modifying geometry, imposing boundary conditions, etc. In other words, stabilization is not meant to be a band-aid for a poorly built model; nor is it meant to simulate general rigid body dynamics. Chatter and contact initialization should also be dealt with by using alternative modeling methods.
Because of the nature of contact stabilization, results really aren’t actually valid until the very end of the step, when all of the damping energy has been ramped to zero and static equilibrium is maintained only by the contact forces that the model was intended to resolve.
But Be Careful…
Check out this perfect example that we recently happened upon when testing contact stabilization on a 4-Point bend test specimen. The model is of a 100mm long, scalloped steel test specimen that is 10mm wide and 3mm in height. The rollers are rigid bodies and are 10mm in diameter. The arbitrary (made-up) stress-plastic strain curve can be seen below and is reasonably representative of mild steel.
As can be seen from the plot below, the addition of contact stabilization to this problem has a significant effect on the results in the early stage of the analysis. This could result in significant overprediction of strength of the component if we didn’t know what we were looking for or if results were blindly trusted with minimal attention. As we can see, the lines begin to converge and the solutions predict identical forces by the end of the step - once all of the damping energy has been removed.
This is by no means fully representative of the behavior you’ll see every time you use contact stabilization, rather a cursory word of warning. It is possible that convergence between the lines might not happen until the very end of the step, or even fail altogether (which usually results in failure of the step to fully complete in the ‘no stabilize’ model). Imagine your customer or boss asked you what the ultimate strength of this bar was in four-point bending and you told them 376N… You might be in big trouble when the physical test came back less than half that value.
Final thoughts
Contact stabilization is a very important weapon in the FEA analyst’s arsenal, but it must be used with caution. If rigid body motion is going to be a concern early in the analysis step, but is expected to be resolved by the end, then it is absolutely necessary to stabilize the contact. However, please do keep in mind that, as we saw in our 4-point bend test example, the results at any point other than the very end of the step can be unreliable and should at least be carefully scrutinized before being reported.
If you need help with FEA or simulation in general, don’t hesitate to give us a call! We’re passionate about what we do and we’d love to learn more about your unique engineering challenges.