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nano model structural

I want to analyze this model that is stressed and deformed. I fixed the lowest of points in the x, y, and z directions, and I pulled top of points upward (y direction). When I run the model in Ansys I got the following error:

Check for insufficiently constrained model.

enter image description here

I tried to cut the model to make it smaller, and analyzed the model like this. I fixed all points in the x and z directions. The lowest of elements are all fixed in x, y, and z. This didn't work.

enter image description here

I then cut the model to one element. It worked. enter image description here

  • How can I analyze this model?
  • What is the error and how do I need to set the boundary conditions?
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  • $\begingroup$ It's hard to see exactly what you have released in your model (on my phone screen, at least). Only thing I can think of is that you might have the vertical pieces torsionally released where they connect to each other? $\endgroup$ – grfrazee Aug 2 '15 at 15:49
  • $\begingroup$ Also, it would help if you could zoom in on Node 63, which is where your analysis is detecting the instability. $\endgroup$ – grfrazee Aug 2 '15 at 22:00
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    $\begingroup$ I can't see anything wrong in your model. Try running with, say, two connected hexagonal cells rather than a complete ring. $\endgroup$ – rdt2 Aug 5 '15 at 10:15
  • $\begingroup$ What to do is run a modal simulation and see what degrees off freedom are there you did not expect. $\endgroup$ – John Alexiou Mar 5 '16 at 23:25
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It looks like your top points are not constrained in the X-Z plane like you've claimed. This would allow each top point to swing back and forth towards the and away from the center of the ring.

As for the original model, perhaps your bending stiffness is to low. If there is no bending stiffness that would also explain the problem. Even if in reality there was no bending stiffness and the mesh deforms just find without coming apart, numerically the solution could be unstable. Imagine two stiff links with a hinge, one end anchored, and pulling on the other. Obviously the solution is that the links will be pulled straight, but numerically, the center joint could bend slightly back and forth and the restoring force would be tiny, so it would probably not converge.

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