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Calculate force required to crush a tube
5

Calculate force required to crush a tube

Calculate force required to crush a tube

(OP)


I have some tube (see above & ignore the fact that it's bent for now) and I'm trying to find out how I can calculate the force or load it'll take before the tube crushes.

I can't seem to find this anywhere but I think I might be looking in the wrong places. I've found plenty of buckling formula but I'm almost certain that those are either for compressive strength or for buckling along a given length. I think what I need is much simpler, but I'm totally lost on this one. I'm hoping someone here might be able to point me in the right direction.

I'm assuming that realistically I should only need the cross-section properties and material properties to be able to figure it out but at the moment I cannot see the light.

Thanks in advance.

RE: Calculate force required to crush a tube

This is one I'd tackle with a non-linear FEA.

RE: Calculate force required to crush a tube

(OP)
See that's where I fall down. I can use Inventor for FEA but not only do I not trust it, but I'd also like to be able to figure this one out manually.

I'll google non-linear FEA and see if I can find anything though. Cheers.

RE: Calculate force required to crush a tube

Nonlinear FEA will be the most accurate but you can try solving this as curved beam. Check Roark’s Formulas for Stress and Strain.

RE: Calculate force required to crush a tube

(OP)
Thanks. I'll look into them both. Think I've already been via Roark's formulas but I'm not sure they were what I was looking for.

RE: Calculate force required to crush a tube

Check the chapter 9 "Curved Beams". In the text and in the table 9.2 ("Formulas for circular rings") you will find necessary equations.

RE: Calculate force required to crush a tube

just a bit different approach (excerpt from "Structural analysis of buried pipes" - Hornung/Kittel - Bauverlag - 1989







Further details and explanation in "Structural Mechanics of Buried Pipes" - Watkins/Anderson - CRC Press - 2000

RE: Calculate force required to crush a tube

Roark, 7th ed, table 9.2, case 1 ... Mmax = 0.3183*MR*(1-a), a, alpha, = I/(A*R^2) = (rho/R)^2
compare to plastic bending allowable.

Answer is IMO approximate as it is not designed to answer the question "load to crush the tube", though once the critical section goes plastic ... the end is nigh.

Do you have a press ? why not "suck it and see" ?

another day in paradise, or is paradise one day closer ?

RE: Calculate force required to crush a tube

(OP)
Thanks, everyone. Lots (more) to look at.

rb1957 I don't have a press local to me atm as I'm working from home :'(... But might be something I can look into once Covid has buggered off enough that I can return to work.

Cheers.

RE: Calculate force required to crush a tube

I think you need to define what it is you're trying to solve first as it's not clear.

Is this a uniform external force like being submerged at the bottom of the ocean with only 1 bars inside?

Is it a long piece or a short piece where the tube is kept open?
Is it a span between two point supports or uniformly supported on one side (like being laid on a flat plate or a concrete pad.

Is it a point load?
Is it a point load one side and a flat plat the other side?

Is it being crushed by two flat plates ( like a vice?)

does it have any other bending forces going on ? Or axial or compressive forces?

Does it have internal pressure?

Is there ANY existing ovality

Only when you've figure out all that can you try and work anything out.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Calculate force required to crush a tube

(OP)
LittleInch. There's actually a fair bit going on with this, I was trying to simplify it and break it down into smaller calcs in the hope of getting a rough idea if my design is suitable or not.

The whole thing looks like this...


The SHS is basically a spreader beam which I am happy is strong enough based on my calcs of that as a spreader beam. The pipe is acting as a large rope/hook loop basically. It will see load on the underside of the top bend. As the top is formed obviously it will have ovality already.

I'm fairly confident that the pipe in tension is more than strong enough so really was just wanting to figure out if it would crush at the top where the point load will be.

RE: Calculate force required to crush a tube

this is much different to what I thought you were doing (I thought you wanted to crush the tube).

bending tube is quite specialised, if you want nice looking bends. If think you want some sort of pipe-bender ... like maybe what they use for electrical conduit ... plumbing feels too heavy duty.

Sorry ... must read the post ... fully !
The maximum load on the bend would be a function of how concentrated the load is, and how you define maximum ... when the part is starting to deform, or when the part is in two pieces.
Then of course the useful load would depend on the SF you apply to this allowable load.
I guess you could develop a moment in the bend from symmetry, and so bending stress in the pipe ?

another day in paradise, or is paradise one day closer ?

RE: Calculate force required to crush a tube

(OP)
No, I'm not trying to bend the tube, the tube will be bent already. I'm using the bent tube to lift a load using the above, welded assembly.

RE: Calculate force required to crush a tube

I thought as rb1957. To me you have to consider the shear stress of the lifting load (in plane of the circular section) plus the longitudinal stresses (orthogonal to the circular section) : may be you can super-impose two conditions as per Roark's formulas in table 13.2. Good luck (by hand calcs)

RE: Calculate force required to crush a tube

Why not use solid bar?

----------------------------------------

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

RE: Calculate force required to crush a tube

Oh man, that's a FEA job. all other calculations as realtively simjlistic and amke some assumptions about the tube that you don't have.

Also there a massive difference between a rope / sling which is a distributed load and "hook" which implies really high point loads.

But I don't know why you're bothered. if the tube flattens then so what? You haven't got flow through it and it won't break because of that. Your failure modes must surely be in tension failure of the tube or failure at those little 45 degree elbows. I don't know why you don't just weld the tube at an angle and forget about those little bends with their stress concentration factors.

If anything a little bit of flattening at the hook point could be a good early warning that your tube is overloaded...

This really just goes into the test it to destruction mode then apply a SF of about 3. IMHO.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Calculate force required to crush a tube

FEA ? "surely" the upper curved piece is symmetrical, so consider 1/2 as a cantilever ... a curved cantilever may be, but still something we can approach with hand calcs ?

sure this isn't looking at collapse of the CL section but it should be at least close to the answer ?

another day in paradise, or is paradise one day closer ?

RE: Calculate force required to crush a tube

(OP)
Thanks again, everyone.

dgallup, only reason not to use solid bar is to save weight. This lifting bar will be bolted to something that floats.

I knew it was going to come back to FEA. Looks like I'm going to need to get better at hand calc FEA. Something I've not done before.

LittleInch, the tube goes through the box and is fully welded top and bottom to provide greater strength. Though I understand what you're saying, the loads this is taking aren't really sufficient enough to worry about those stress points.

RE: Calculate force required to crush a tube

Could you pack the inside with sand?

Or epoxy and let it go off?

Or grout? Or sand and cement and water

You only need to fill the small bent part at the top.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Calculate force required to crush a tube

if you want a "proper" loadpath between the bent beam and the box section ... you could put a through bolt.

is the question about the structure or about making the curved tube ?

I suspect that the weakest link is the welds.

another day in paradise, or is paradise one day closer ?

RE: Calculate force required to crush a tube

Our lifting eyes on spreader beams were always solid bar. A hook can deliver a very localized load, so where a sling would work just fine a hook could lead to collapse and disaster.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

RE: Calculate force required to crush a tube

I don't think FEM will help if you don't know the build-in stress due to bending the tube/bar, and the deformation of the shape. but I believe there are available information provided by the manufactures on the web, if you know how to search.

RE: Calculate force required to crush a tube

I reached out to a former co-worker to mentioned that we did use pipe for some of these. We would use 1.25sch80 pipe (1.660" x 0.191") and then on the inside of the top, the lifting area, we would double it with a saddle made from a split piece of 1.5sch40 that was bent to fit. If you use nominal sizes the ID is too small, but pipe is always made to the thinnest wall allowed. These would be blacksmithed to fit and then welded in and they would extend down into the straight portion a little ways.
This would take our wall from about 0.170" to about 0.300" for some local reinforcement.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

RE: Calculate force required to crush a tube

Load test a real sample.

If this is a spreader beam it is supposed to be load tested prior to being placed into service.

RE: Calculate force required to crush a tube

(OP)
Thanks again everyone for your input on this.

It might be worth mentioning materials at this point. This is stainless steel (316) 60 mm x 5 mm SHS & 1 1/2" SCH 80 pipe. It's a beefier version of its predecessor which was only made from 1" SCH 40 pipe, however, the span has grown to 1200 mm and the weight doubled since then to about 500 kg, hence the upgrade. Oh and the bolt plates are 10 mm.

Old for ref:


I quite like the idea of making the top of the loop solid but I'm still not sure if I'll need too because, in reality, the load isn't that huge.

I'm gonna have to run an inventor FEA aren't I? haha

RE: Calculate force required to crush a tube

(OP)


Min. factor of safety of 4.65 which is at the connection between the pads and the box section. This could be an artefact, but regardless, this isn’t bad at all!

RE: Calculate force required to crush a tube

Well 500 kg isn't to be sniffed at and for me the key point is what is the contact area between the lifting device and the pipe - a small hard contact from a hook, metal chain or metal rope or a larger uniform load from a sling or plastic rope?

but yes, 1/1/2" sch 80 - Is this C stl and not stainless? - seems pretty beefy.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Calculate force required to crush a tube

(OP)
Yes, that was my concern and it could easily see a knock as it's industrial equipment.

Not carbon steel no, 316 stainless.

RE: Calculate force required to crush a tube

If you have not already done so, you should review

ASME BTH-1 Design of Below-the-hook Lifting Devices

ASME B30.20 Below-the-hook Lifting Devices

RE: Calculate force required to crush a tube

(OP)
Thanks, will take a look.

I've actually done quite a lot of work on lifting lugs and created a pretty comprehensive spreadsheet for that using ASME BTH-1 calcs. This thing is a bit of a different animal though, haha.

RE: Calculate force required to crush a tube

If that's true schedule 80 (5mm) then you actually don't have a lot of force trying to crush the pipe.

the forces are getting distributed all over the place at the top of that triangle and hence I suspect something else will buckle or break before your tube gets crushed.

Let us know when you do the destruct trials what happens....

I assume this will be proof tested to a FOS of 3?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Calculate force required to crush a tube

I don't see a solid bar as a weight penalty. You gain nothing by using a tube. This isn't a space frame where the increased moment of a larger diameter hollow tube improves stiffness.

----------------------------------------

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

RE: Calculate force required to crush a tube

(OP)
@littleInch, thanks, will let you know how it goes. We usually only test to 2x but usually do calcs to 5x, go figure...

@dgallup, you're actually not wrong. Might be worth considering. I'll have a ponder over the weekend and come back to it. Cheers.

RE: Calculate force required to crush a tube

if you did make the bend from barstock, it'll take a lot more force to bend ... you make need to stretch and bend ?

you'd probably also need to "counterbore" the ends where you weld to the tube.

I think the welds are the weak link, and that tube is more than enough (compared to the welds).

another day in paradise, or is paradise one day closer ?

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