Solid vs Hollow

[OutCell]

Hi, Why is hollow tubes stronger than solid tubes? And why is circular tubes weaker than hexagonal ones? Thanks

[tkiwan]

Hollow is stronger because of surface area but it changes with wall thickness & steel grade.

[winnie]

If its made of the same material then soild is stronger than hollow. Circular is stronger than hexagonal providing same material and thickness.

[Cryos]

Not true, holow allows it to flex and distribute the impact across the surface. Solid focus's it in the central of the impact.

Its the reason why chassie legs on cars are hollow and not solid, it is also why on a rollcage that has a rectanular flat area why holes are cut, to give it strenght and flexability.

[Tim Falce]

This may be better in Race tech where I am sure the resident geeks will be able to put you all right

[OutCell]

Thanks for the replies people but some answers confused me.. I tested this on a CAD package with FEA.. The hollow is always stronger than solid (Under my certain boundary conditions).. And when i analyze surfaces only, hexagonal shapes are always stronger than circular ones.!! Am i wrong on my analysis?

[winnie]

If you fixed a piece of identical diameter solid and hollow tube at either end and hung a weight in the middle until the tube collapsed - the solid would take more load.

If chassis legs were solid then it would add a huge weight to the car. Holes are cut in strengthening plates to save weight, not to make it stronger.

Hollow tube's strenght against weight is much higher than soild tube / bar.

Why arn't con rods, crankshafts, bolts or wheel studs hollow.

[Cryos]

Lets put this in racing tech!

[tristancliffe]

Correct.

Solid is ALWAYS stronger that Hollow if material and outside dimensions stay the same. But if you increase hollow diameter so that is is the same stiffness as the original solid it will still be a lot lighter.

Your FEA analysis is wrong somewhere. Hexagon bars would be weaker than round bars (assuming that the hexagon is inscribed within the diameter). Even if it was inscribed outside the diameter the edges would probably act as stress raisers anyway...

@winnie - if you want to be pedantic, then conrods and crankshafts etc often are hollow, even if it's only very small IDs for oil transfer

[OutCell]

This is getting real interesting :P And i keep thinking that i messed up my final yr report... Now in this picture http://www.lnengineering.com/fe.jpg The 5.50E+04 , means it's withstanding strength right? So if i get a second figure which is bigger it would mean that the second figure is stronger right?

[tristancliffe]

Erm, no.

That image shows the von Mises stress of the part, given the loadings and mesh structure you've used (and possibly heavily altered by whatever constraints you have used on the part).

A bigger number would be more stress, so for a given material more likely to fail (ignoring fatigue). At the moment your peak stress is 5.5E4, which may or may not be near the yield stress of your material. If it isn't good enough (too likely to fail, stretch, whatever) then you can either lower the stresses by revising the design (a bigger radius on the big end?) or using a better material (or better heat treatment, or better shot peening etc).

If the second figure is bigger then you have MORE stress, and the part is more likely to fail.

There is no such thing as "Withstanding Strength".

What does a conrod have to do with solid/hollow bar discussions?

[OutCell]

Originally Posted by tristancliffe

Erm, no.

That image shows the von Mises stress of the part, given the loadings and mesh structure you've used (and possibly heavily altered by whatever constraints you have used on the part).

A bigger number would be more stress, so for a given material more likely to fail (ignoring fatigue). At the moment your peak stress is 5.5E4, which may or may not be near the yield stress of your material. If it isn't good enough (too likely to fail, stretch, whatever) then you can either lower the stresses by revising the design (a bigger radius on the big end?) or using a better material (or better heat treatment, or better shot peening etc).

If the second figure is bigger then you have MORE stress, and the part is more likely to fail.

There is no such thing as "Withstanding Strength".

What does a conrod have to do with solid/hollow bar discussions?

I needed some sketch of an analysis to understand the figures i have correctly.. So you are saying that the bigger the Von Mises stress, the more faster it would fail? Does it means the limit at which it would fail?

[tristancliffe]

Take a ruler. Pull it. You have put a stress on it (e.g a von Mises stress, if you like).

Pull it harder. Your stress just got bigger, and it might break.

Pull it as hard as the first time, but this time put a nick in the edge about halfway along. Same force at each end, but higher stress in the middle, so much more likely to fail.

Notch sensitivity is major thing with engineering design, and reducing notches, stress raisers and all that sort of thing is what FEA is meant for.

The limit at which it would fail is when the peak material stress exceeds the material capacity, be it in shear or tension.

But still, what does this have to do with hollow/solid bars? You should be asking pertinent questions about second moments of area really, and doing some sketches/sums on a handy napkin.

[OutCell]

Originally Posted by tristancliffe

Take a ruler. Pull it. You have put a stress on it (e.g a von Mises stress, if you like).

Pull it harder. Your stress just got bigger, and it might break.

Pull it as hard as the first time, but this time put a nick in the edge about halfway along. Same force at each end, but higher stress in the middle, so much more likely to fail.

Notch sensitivity is major thing with engineering design, and reducing notches, stress raisers and all that sort of thing is what FEA is meant for.

The limit at which it would fail is when the peak material stress exceeds the material capacity, be it in shear or tension.

But still, what does this have to do with hollow/solid bars? You should be asking pertinent questions about second moments of area really, and doing some sketches/sums on a handy napkin.

Well its for a roll cage design, i have to use FEA.. What can you say about this picture http://i169.photobucket.com/albums/u...viewtubing.jpg in relation to strength if possible ..

[tristancliffe]

They're all in compression, right?

The hexagon has the highest FEA von Mises stress and, for a given material (4130?) will fail first, by quite a big margin.

Your square tubing has the lowest FEA von Mises stress, and is therefore least likely to fail under those conditions. However, you can see just from looking that the square section has a bigger area (are they the same wall thicknesses?), and would therefore have a lower stress concentration.

Have you looked at real, professional roll cages? They are ALL round section, and not just because they happen to have a lot of spare round section tube they didn't know what to do with.

I'm guessing they are in compression judging by the bending style. Presumably you are analysing above the Euler bucking load for each profile:length?

[OutCell]

Originally Posted by tristancliffe

They're all in compression, right?

The hexagon has the highest FEA von Mises stress and, for a given material (4130?) will fail first, by quite a big margin.

Your square tubing has the lowest FEA von Mises stress, and is therefore least likely to fail under those conditions. However, you can see just from looking that the square section has a bigger area (are they the same wall thicknesses?), and would therefore have a lower stress concentration.

Have you looked at real, professional roll cages? They are ALL round section, and not just because they happen to have a lot of spare round section tube they didn't know what to do with.

I'm guessing they are in compression judging by the bending style. Presumably you are analysing above the Euler bucking load for each profile:length?

Compression yes. DAMN I AM SCREWED.. I showed the person in charge of my report these and told him that hexagonal would be stronger and he SAID NOTHING.. I got everything wrong and the other way around... DAMN IT, I DONT WANT TO FAIL MY FINAL YEAR Sorry but i am really ****ed and about to kill myself

[tristancliffe]

All I can say is ha ha ha. You should pay attention in school. I don't think you'll kill yourself - there are always other careers!!!!
FEA shows the component stress not the part strength.

[GORDON STREETER]

Originally Posted by winnie

Why arn't con rods, crankshafts, bolts or wheel studs hollow.

Fords tried this in the 60s . As most Ford buffs will tell you the cranks snapped like carrots, especially the 1340cc Classic one and I broke loads !

[dtype38]

Outcell, I'm afraid you don't seem to understand the subject of stress in hollow and solid sections well enough yet to compose a sensible question about it. May I suggest you go and study a few very basic text books on the subject, which I'm sure you would have for your course. Might I even suggest you ask your lecturer to go over it with you. In my experience they are just as keen for you to pass your exam as you are. When you've tried those you're welcome to ask for further advice. Good luck.

Thread Closed.