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# How to Constrain Hollow Tube under Thermal Expansion

## How to Constrain Hollow Tube under Thermal Expansion

(OP)
When applying a global thermal load to a hollow tube, the tube wants to expand radially so how do you constrain the tube without restraining the expansion.

I'm simulating a tube under pressure and temperature and fixed at both ends, but if I fix both ends I get extremely high stresses at the ends.  The tube has been idealized as a shell.

KL

### RE: How to Constrain Hollow Tube under Thermal Expansion

KL,

I recommend using a cylindrical coordinate system at each end. Use this local coordinate system to create the constraints in the z direction only at each end. This will allow for the radial direction to be left free. Be careful not to under constrain your entire model, however, you may need at least one point to have a fixed radial DOF. Always check your constraints for near zero reaction loads to ensure your model is in static equilibrium.

Best regards,

Matthew Ian Loew

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### RE: How to Constrain Hollow Tube under Thermal Expansion

(OP)
Thanks Matthew.  How do u add cylindrical coordinate in Wildfire?

KL

### RE: How to Constrain Hollow Tube under Thermal Expansion

I don't have MECHANICA open in front of me now, but you should be able to just add a coordinate system of that type. Use the help functionality for specific questions like that.

Best regards,

Matthew Ian Loew

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

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