Very often when encountering problems with a simulation, it is all too easy to look at the simulation inputs for a solution. For example, if the model doesn’t mesh or solve you immediately think, “is my mesh too coarse?”, “are all the parts bonded correctly?”, or even “am I using the right solver?”.
However one very important variable in the success of a simulation is the CAD model itself, which becomes very sensitive if it hasn’t been built with analysis in mind. A common example of this is when someone gives you a model converted from another CAD source or from someone who has no idea that simulation will be done on their assembly.
I came across such an example last week when working on a file that had been converted from another CAD package. The model seemed to mesh ok but when I came to solve it I got an error that read:
Now the solution to this problem is usually to do with parts not being properly restrained (as it says), leaving rigid body motion a distinct possibility.
A common next step is to run the same model through a frequency analysis, because this will happily solve for the rigid body modes and help identify where the failed connections exist. However in my model the Frequency Study wouldn’t solve either giving a similar error!
Having checked and re-checked my simulation setup for the correct boundary conditions I ended up back at the geometry, which is where the problem was finally discovered. Because various CAD software uses different tolerances during CAD creation, an imported file from another source may struggle to define global bonded contacts within SolidWorks.
To check the contacts, use Tools > Interference Detection, pick the assembly and hit the Calculate button. A list of all inter-part contacts should be given that allows you to select each one and check out the contacting areas of your model.
It is useful to note that if you have the ‘Treat coincidence as interference’ ticked and you get a series of interferences, but switching this off reveals no interferences, then the global bonded contact definition should work for you. This was the case I had in my solid model, yet I was still getting a failed solution! However going through this interference list step by step eventually highlighted the problem.
About three different regions displayed a single straight line, rather than an area as the interference region between two parts. This is shown as a thick red line (below), which is much more obvious than the other shaded areas of interference. And no matter how closely you zoom in to this line, it never enlarges to the typical shaded area.
Clearly these thick lines indicate areas of the geometry where bonding is applied
along an edge. This in turn implies a zero area for the stress to be calculated on and so an infinite result would ensue, something we can’t have.
To fix this problem I chose to create a new merged part containing all the separate components in the assembly so that the other touching faces would transmit the load and no bonded contacts would need to be used.
The moral of this story is that doing simulation needs a solid basis to work from. Don’t forget to check the integrity of your CAD model, especially if it was originally built with no concern for simulation.
That's very true, I had similar problems with a part once. I would also like to add that one should try to avoid extremely small edges or weird topology such as tiny faces within faces.
http://img.photobucket.com/albums/v252/Kev_Boy/SolidWorks/Tight_Edges.jpg
http://img.photobucket.com/albums/v252/Kev_Boy/SolidWorks/Small_Faces.jpg
I had to bring in surfacing commands, because a single cut-loft was to complex. You can simulate a complex model, but only if it has a clean topology without weirdness that confused the meshing.
Posted by: Kevin De Smet | 30/09/2009 at 04:32
Good comment Kevin, and good examples, thanks. I have worked with different analysis programs over the years (mostly standalone ones) and one problem that occurred over and over again was the quality of the CAD model. It is very satisfying to be working with an embedded product that gives plenty of correction options to the user to fix these kinds of things so you can move on and focus on the simulation at hand.
Posted by: Tony Eckersley | 30/09/2009 at 06:06