Strange rigid body motion

[slumdogengineer]

Ok I don't really like posting general questions but I am a bit stuck so here goes anyway.

I am modelling a long thin balloon which is situated inside two hoops (imagine two rings on your finger, for example). Initially the balloon is not in contact with the hoops, because their radius is slightly larger than that of the balloon. Then, under internal pressure loading of the balloon, it expands outwards and comes into contact with the hoops.

So I have been trying to use surface-to-surface contact to simulate this (targe170 and conta174 elements). I have placed zero-displacement constraints on either end of the balloon, and at some points on the hoops. Both the balloon and hoops are modelled with shell elements.

Now, when I run the analysis (static) I always get an error message saying that there have been some very large displacements, indicative of rigid body motion. I can't understand this, because of the three parts of my model (balloon, and the two hoops), all are constrained at at least two points, in all DOFs, which should surely prevent this.

So, although I appreciate that is difficult to diagnose a problem with so little information, has anyone had maybe a similar problem with strange rigid-body-motion under contact conditions? Or has any general suggestions?

Thanks for any help,
Will

 

[aminjamali]

Sometimes I get rigid body motion issues in my contact problems even though the structures are statically stable. What I do is to review contact properties and status, applying small load or sometimes even a small value for friction. These might be helpful?

 

[ansysfreak]

Hi

Did you constrain the rotating axis of the ballon and hoops? Because they otherwise can rotate about there own axis.

 

[slumdogengineer]

Thanks for the suggestions. The balloon and hoops are fully constrained, yes.

I'll try playing around with the loading for a while and see what happens. If I solve the problem I will post here how I did it, in case anyone else gets in the same trouble!

Thanks again.