In this month’s tips and tricks video, I walk you through mechanical contact without form assembly mode using COMSOL’s new version 6.3. Our team will use this for any scenario where we have a number of bolted connections or any decohesion problems where there’s not a lot of displacement or rotation. The benefit is that you don’t need to go to assembly mode, and for large assemblies where lots of parts are coming into contact, you don’t have to set up the contact pair definitions. Watch and see
Click below to see why COMSOL version 6.3 is perfect for simplifying your model workflow with the technique presented in this tips and tricks video.
We have included the video transcription below to help you follow along.
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Mechanical Contact Without Form Assembly Mode
Hi this is Josh from AltaSim Technologies, and I’d like to present to you a tip from COMSOL®, and that is in the new version, version 6.3. COMSOL® now supports mechanical contact without Form Assembly Mode – without form assembly mode. So previously, as you may know, on the left is an example of how COMSOL® unions geometries together. So, if you had a block resting on another block within your geometry sequence, and you left the geometry sequence, COMSOL® would Form Union for you automatically and provide a connected single boundary between these two and produce a conformal mesh.
What you need to do is, in order to establish mechanical contact previously, is switch to Form Assembly mode, which would allow for the boundaries to be duplicated and for non-conformal meshing, no points disconnected by default, and then you would set up through contact pair definitions how you wanted these faces to interact with each other. You can see how the triangular elements don’t necessarily line up here because these parts are disconnected. And so, if you ran an analysis where you pulled the block the top block up and off of the bottom block, if you were previously in Form Assembly mode or excuse me if you were previously in Form Union mode you would it would be as if these materials were welded together or glued together. Whereas on the right, you have Form Assembly mode and allows for the parts to separate. So, this would be pulling the top block up relative to the fixed bottom block. The inverse of this would be pushing the top block down into the bottom block. And on the left we have the default Form Union feature. Whereas with the Form Assembly mode and no friction, it’s a more physical analysis. If the scenario was that these blocks are not welded or glued, it would allow for sliding between the bottom and the top mesh. And that’s the standard way of doing analysis in COMSOL® up to this point, if we’re doing contact.
And, but now COMSOL® is supporting a new Interior Contact feature that is a simplified contact formulation. A couple of the advantages are no need for assembly mode, which means no need for contact pairs. And it does not force a geometrically nonlinear study as traditional contact implementation does. A few of the limitations, it cannot be used for modeling dynamic events, and it’s limited to small-scale sliding between the contact boundaries. And so, we’ve we believe here at AltaSim that we will be using this for any scenarios where we have a number of bolted connections or any decohesion problems where there’s not a lot of displacement or rotation. And, because the benefit is you don’t need to go to assembly mode and for large assemblies where lots of parts are coming into contact, you don’t have to set up the contact pair definitions. And so, we ran some, here’s an example of what the model tree would look like when you add in the Interior Contact feature right click Solid Mechanics, mouse over More and add Interior Contact. And it’s going to provide a slit boundary condition that all of these boundaries where the parts touch each other. And this will allow for different, it will force conformal meshing, but it will introduce separate degrees of freedom on the upside and downside and allow for the Standard Penalty and Augmented Lagrangian contact definitions with the simplified geometric setup and simplified meshing strategy. So here was our test with the Interior Contact and again this is Form Union mode but the blocks are allowed to separate with the new Interior Contact feature. And, if you push the blocks down they will slide relative to each other. Now we did notice that we get a little bit of convergence issues with the default solver settings, with Augmented Lagrangian, so we’d recommend going with the Penalty approach that does allow for some penetration between the top and bottom parts for numerical stability. But you can solve this with Form Union feature with Interior Contact now in the new version.
We also ran a test on two steel plates with the bolt connection. Where you have the top plate connected to the bottom plate through this bolted connection this is just a simple demo, but we would recommend using this formulation for large structures that have many bolted connections. So we want a Pull Apart analysis where we mount this side with displacement controlled zero displacement and we pull the geometry apart half symmetric, 800 pound pull apart load, and we’ll just solve this with the standard way. Firstly, as a baseline test that we can get the displacement prediction deformations magnified 65 times to show the shape of the bolt connection is holding the plate together, the two plates together. And we can solve for the stress relative to the yield strength. And in this region near the bolt, we would expect the stress riser 16% of the way to permanent deflection, so we’re still safe from a permanent deflection standpoint. Now we can run the same analysis, so on the left was Assembly mode with contact pairs which was the traditional way and we wanted to test out the new Interior Contact feature to make sure we got the same solution, and we do within 1% of the displacement peak displacement prediction. We also get a test here for the In Contact control variable so in magenta with the standard technique you can track which portion of the surface is in contact. This is the contact between the two steel plates. Now another note here is that we don’t have access to this solid.incontact control variable with the new Interior Contact feature, so we would expect you’d want to do some additional work if you wanted to do any multiphysics couplings; if you going to introduce any thermal contact conditions that might take a little bit more work with this particular feature, but still be doable. We did solve for the contact pressures in both scenarios and we get a similar result here with the magenta regions exceeding 0.5 mega pascals of contact pressure, and very similar pattern between the two techniques. Again, the Form Union mode with Internal Contact is going to be recommended for large models that have a number of bolted connections it’s going to really simplify your your contact paired definitions and your model approach tracking of faces, etcetera. And we’re getting the same stress simulated stress results the two different methods standard method on the right, the new method on the right standard method on the left assembly mode with contact pairs new method on the right union mode though with internal contact. The new method also supports friction, and you can see that here showing just in the wake of the bolt, if you introduce friction with the coefficient of friction definition you can track the normal, but also the tangential forces that would develop between the top and bottom plate and similar result here as well.
So, our takeaways, back to the opening slide here, the takeaways that enter your contact new in version 6.3, we do recommend using it if you’re doing scenarios that are not dynamic events and only have small scale sliding, particularly with bolted connections. We’re looking forward to being able to simplify our model workflow with this new technique. So hopefully you found this tip helpful, we’d be happy if you reached out to us if you have any questions on this particular tip or anything else related to your modeling and COMSOL®. Thank you.