Tube stiffness perpendicular to it's axis
Tube stiffness perpendicular to it's axis
(OP)
I need to design a die that will allow me to crush a 3.0in. diameter 0.096in. wall 6.0ft. long DOM tube into an oval ~1.75in. x 3.XXin.
Can someone point me towards something that will teach me how to calculate a close estimate of the stiffness of a cylindrical tube perpendicular to it's axis. In addition links to any websites that would give me any additional info would be greatly appreciated
I don't have access to any FEA software, or testing equipment. :(
Thanks for any help you can give,
Chris Skarzenski





RE: Tube stiffness perpendicular to it's axis
RE: Tube stiffness perpendicular to it's axis
This response and the one above are both assuming that you apply pressure along the whole length of the tube. If you are applying a point load, and just crushing the tube in one place, that would be different.
Also, when you compress the tube, there's no guarantee that it will go into a nice oval shape- at some point, it would deform into an 8-shaped cross section, sort of.
RE: Tube stiffness perpendicular to it's axis
If you fill it with sand or salt and weld the ends closed, you can probably crush it in a big press brake with a flat punch and a flat die. Better and probably safer if you have a flat urethane die. Restrain it so it doesn't pop out.
You might be able to crush it to a defined shape with multiple passes through a two-roll forming machine. The required custom rolls would be a bit cheaper than a press brake punch and die.
Mike Halloran
Pembroke Pines, FL, USA
RE: Tube stiffness perpendicular to it's axis
However, the simplest mechanism that can give plastic collaps is a 4 bar link, so you can work out the Force required to fail the metal at 4 plastic hinges by using a work equation and the fully-plastic moment of the wall.
In practice I suspect this is an upper bound estimate.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Tube stiffness perpendicular to it's axis
RE: Tube stiffness perpendicular to it's axis
Dik
RE: Tube stiffness perpendicular to it's axis
Thank you everyone for your help. I have ordered Roark's book and it will be here Thursday.
I will only be making the tube in 12ft section two to four times a year so anything excessively expensive or overly complicated is not desirable. The two roll forming machine is a good idea and I will look into doing rolls instead of dies.
Greg,
I am interested in the method you mention but I am not familiar with it. Can you recommend some good reference materials?
Once again, I greatly appreciate everyones help.
Chris Skarzenski
RE: Tube stiffness perpendicular to it's axis
dik
RE: Tube stiffness perpendicular to it's axis
The fully plastic moment of a hinge in tube of length L is Mp=yield stress*t/2*2*t/4*L, I think
Then set up a mechanism that will collapse, work out the angular deflection of each hinge, for a given deflection at the loading point delta, and its length. Use Mp as above.
Then you know F*delta=sum(Mp*theta) for all hinges
Choosing the appropriate mechanism is the trick - you can get very clever and differentiate a parametrical shape to find a minimum for F, but your 3d geometry better be really good.
Alternatively 20 minutes in the workshop with a hydraulic press would probably pay dividends. Grin.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Tube stiffness perpendicular to it's axis
RE: Tube stiffness perpendicular to it's axis
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Tube stiffness perpendicular to it's axis
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.