FEA constraints - pressure vessels

[Cnoj]

I'm modelling a pressure vessel that will have no forces applied to it other than pressure and the basic shape (for now) is just a hollow cube.

What's the best way to constrain the vessel so I can apply equal pressure to it's 6 surfaces? It seems like I shouldn't need any constraints as the system would be in equilibrium without them but Inventor doesn't agree with me.

I'm using the basic FEA package included with Inventor 2008 Pro.

[JDMather]

How will the part be constrained in space in the real world? Mounting brackets? Gussets?

[blair]

When you transfer models into FEA, they come into a frictionless and gravity free zone/state. Your box-tank would need some form of support in real life, even if it was sitting on a table. I would probable use the split-face tool to create a couple of areas on your base to place supports. I would fix only one of the areas (XZY) and then place single direction constraints in the direction of your gravity. I would enable Gravity in your model as well. These area points may be small spots on the bottom corners of your model.

[Cnoj]

Gravity. it will sit on the floor and the base will be free to deform.

[blair]

That's what I thought, follow my instructions for the constraints, use very small "split-face" in each corner of the base. This will allow the center of the tank base to be free to deform under load.

Fix the one corner in all axis, then support/fix the other three corners with only a single axis constraint. If your tank base plate is in the XY plane, then support the three corners in the Z only.

[Cnoj]

I don't have access to the machine with Pro on it right now but from memory it only had 3 constraint types, Frictionless, Pin (Roller maybe?) j and Fixed(?). I'm probably mis-remembering these but I played with all three and couldn;t find one to suit. Frictionless allowed a surface to slide on a plane but not deform away from it at all. Are you suggesting I use this on tiny feet and if so will the split face allow the rest of the base to deform independently of the feet?

Also when you say single direction constraints for three of the feet, do you mean constrain them to move only along a (vertical) line? It seems like this would compromise the results as the corners of the cube would probably want to move inwards under pressure (it's a vacuum chamber).

Thanks for taking the time to answer,

Jon

IV 2008 pro

[Cnoj]

Blair,

I just saw your second post. I'll take a look tomorrow and report back.

Thanks again,

Jon.

[blair]

If you tank base is in the XY, that mean the Z is the height of the tank. By fixing the one corner in all (XZY) and then fixing the other three only in the Z. It will allow the three corners of the tank to slide in the X and Y (simple support).

[Cnoj]

OK, I eventually found the costraints you described, and got good results.

For ref. : 'Fixed Constraint' has a '>>' button that gives seperate control over x,y & z instead of defaulting to all three at once.

Thanks.

[blair]

Sorry about that, I asumed you knew about that when I was describing the constraints. I been using FEA since the early 1990's

[Cnoj]

How do you enable gravity btw? Couldn't find that either.

[blair · Accepted solution]

It's in the "Loads" panel, look for the "Apple" icon.