Comparison of solid and hollow cylinder

[physea]

Hello!

We have a solid cylinder and then we take that and drill its core so that it becomes hollow.

In which case the cylinder will be stronger?

Thanks!

 

[nasu]

What do you think? And in which way "stronger"?

 

[physea]

Stronger meaning harder to bend.

I am not sure, I would assume the solid is stronger, but I need to prove it.

 

[nasu]

Why would it be stronger? It may be almost as strong as before, considering that during bending the strain in the median layer is minimal.

I think you are confused about the practice of using pipes instead of full rods in mechanical structures.
The idea is to take into account the weight of the pipe too. For the same weight per meter, a pipe may be stronger than a full rod. But the rod will have a much smaller diameter than the pipe, in this case.

 

[physea]

I understand what you say. I agree that hollow pipes maybe stronger per weight or that thinner rods maybe as strong as hollow pipes. But I want to compare cylinders of the same outer diameter, one hollow and one not. And of the same material of course. And regardless of their weight/mass, yet the hollow cylinder of same outer diameter will of course have lower mass/weight.

 

[nasu]

Then I see no reason to expect that the hollow cylinder will be stronger. Maybe you can explain what makes you think it may behave this way.

 

[physea]

Some stuff I read online like this http://physics.stackexchange.com/qu...r-than-solid-bar-of-same-outside-diameter-o-d

 

[nasu]

Aha, the very last answer in that thread may have something, regarding the difference in "buckling" behavior.

 

[physea]

I still cannot understand how the hollow will be stronger. That answer you pointed me, says that the solid rod would be slender. By slender I understand it will be thinner. But the question says that both the hollow and solid rod would have the SAME outer diameter, so what is he talking about?

 

[physea]

Any idea?

 

[Nidum]

Do the sums .

 

[physea]

Do the sums .

which sums?

 

[Nidum]

Do you know how to work out the stresses and deflections for a loaded beam ?

 

[physea]

Not really, but I just want a yes or no answer for the beginning, then the why can follow

but I think stiffness is the youngs modulus times the second moment of area

 

[PhanthomJay]

I still cannot understand how the hollow will be stronger. That answer you pointed me, says that the solid rod would be slender. By slender I understand it will be thinner. But the question says that both the hollow and solid rod would have the SAME outer diameter, so what is he talking about?

I don't know what he is talking about. The solid rod has a greater moment of inertia than a hollow rod of the same outside diameter, and therefore can take greater bending moment than the hollow one. It also has a higher buckling load . Both max moment and critical buckling load are a function of I . Now it is assumed however that the weight of the rod in both cases is small in comparison to other applied loads.

 

[Nidum]

For a round bar of steel carrying simple static loads then it is certainly not true .

There are a few situations though where removing metal can make a component stronger .

This has to be looked at sometimes in problems where stress concentration , fatigue , crack propagation , dynamically induced stress or thermally induced stress determines the effective strength .

In these cases 'strength' usually has to be defined in terms of safe service life under a given loading rather than as simple brute strength .