In the last three versions of SolidWorks, there have been some handy enhancements to the ability to assign different treatment of bodies in the Materials folder for Finite Element Analysis. If you haven’t explored what’s under your Right Mouse Button lately, you should do so now!
Open a multi-body part, or an assembly with at least 2 parts in it, and create a Simulation. Click to expand the PARTS folder, and then then right-mouse-click over any of the solid material bodies within.
The additions I want to call attention to, are the choices EXCLUDE FROM ANALYSIS, MAKE RIGID, and FIX. My goal in pointing these features out is two-fold – I want to save you some time and effort if you did not already know about these new options. But also, if you DO already know, and use these options frequently, I want to save you some frustration specifically when setting up a study with Contact.
OK, first the option to EXCLUDE FROM ANALYSIS. You no longer have to create a new Configuration for setting up a simulation, and then SUPPRESS all the solid/surface bodies that are not actually part of the test – you can simply turn them off from here. I find this very helpful as I tend to use a lot of Surface bodies as construction elements and re-usable references. When you are looking at the regular SolidWorks feature tree, the excluded bodies will show normally. But when you switch into the Simulation study manager, the Excluded bodies automatically HIDE.
The two options I wanted to discuss in detail are MAKE RIGID and FIX. These are both handy if you have a part that comes into contact with a lot of other parts, helps hold an assembly together, but it is NOT the focus of the study and can be assumed to be pretty darned stiff compared to your simulated components. Instead of suppressing such a component, and then hunting around to create appropriate boundary conditions on every face of every part that would have contacted it, you can simply FIX the part, and let the neighboring parts bond to it.
MAKE RIGID is even more useful, because the part will behave as infinitely rigid, but it can still move, translate or rotate, as a result of the deformation of any of the parts it is bonded to. This saves you the time and tedium of hunting around for every possible pair-wise set of faces that would have moved in concert with each other, and creating a RIGID CONNECTION between every such pair.
OK, now that we’re all up to speed, for some inside scoop. The way that a FIXED or RIGID body gets treated, is that only the outermost faces get meshed, so in essence the part becomes a Surface body. All the nodes of the surface remain perfectly rigid relative to each other. And here’s why that is important. I’ve had a lot of customers calling in to say that they are evaluating CONTACT between some flexible body, and a relatively rigid part, and so to save time they had set the stiffer body to be FIXED, (or RIGID, either one). Then they notice that their contact forces are either zero, or way off, and that their reaction forces do not sum up to zero across the problem. What gives?
The problem here is that the way we treat RIGID/FIXED bodies is incompatible with the way we create “Gap Elements” to simulate contact. Each gap element is like a conditional, 1-D spring between opposing nodes (or element faces), and it REQUIRES some degree of compliancy, ie some spring-iness, between both contacting bodies so that it can compute the equilibrium position of the line-of-contact. Both sides gotta have some give. Rigid bodies got no give. So the side of the contact that has some compliancy ends up reporting either zero pressure, or some crazy value because it is pro-rating between two effective Young’s Modulii, when in fact one side’s modulus is infinite.
So, don’t do that. The work-around is pretty obvious, once you understand the way rigid bodies are implemented. To check for contact where one member is a FIXed or RIGID body, you have to create another solid body laminated onto the Rigid one, just in the area where the parts will bear on each other. This ‘contact scab’ cannot be a Surface, it must be a solid – it has to mesh at least one element deep – so that it will have some real elastic give to support the Gap Element’s equilibrium equations. Give it some very high Young’s Modulus – I usually make it about 10x that of my stiffest other component in the study.
With that little bit of extra set-up, you’ll have a study where very large, but effectively rigid, parts will mesh as cheaply as possible, and you’ll still be able to conduct bearing or sliding contact checks against them, that will report good contact pressures and accurate force-balances.