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Hello all,

I have searched the forum but I can't say that I am entirely clear on the matter:
I am trying to measure the bend and twist along an anisotropic cylinder after applying a force on its surface along its axis. I have written a code that calculates this using the deformations output (u,v,w) from COMSOL at three points.
As I am getting inconsistent results, I am wondering whether I am using the right coordinates.

1) I only care for the cartesian coordinates, so I'm using a base vector coordinate system with x1,x2,x3 set to 1. I believe that in this way it is the same as the standard global coordinate system so that step was not really necessary, please correct me if I'm wrong.
2) I have tried both with spatial and material frame types on top of the solid mechanics menu, and I get different answers for u and v along the cylinder. Changing the frame in the solutions only changes the display so since I only care about the u,v,w I don't mind so much. The images when I use the material frame are always more sensible (i.e. the cylinder has deformed and you can see the original outline where you'd expect it).
So which one is the correct one to use on top of solid mechanics for my case?
3) Since this is a cylinder, is there a way of simply getting the resulting twist by printing the angle in cylindrical coordinates? Because I have not found how. I tried to define the base vector system in cylindrical coordinates but I couldn't find a way to output r or theta along the rod after the stretching of the cylinder. Do you know a way to do that?

The inconsistency that I mentioned appears when I try the same experiment with 2 different elasticity matrices, which correspond to two exactly equivalent faces of the crystal. (rotated by an angle along z). Since the applied force is along z and all around the cylinder, the resulting twist should theoretically be identical, but it isn't.

I have tested very high (low) tolerance and an extremely dense mesh, so I know this is not numerical accuracy, so the next thing to check would be the coordinates, and I can't say I fully understand the differences between the spatial and material frames.

Any insight would be much appreciated!

Thanks,
Alex