Bending pipe - compression and tension

In summary, the neutral axis of a bent cylindrical pipe will be in the middle, with material on one side in compression and material on the other side in tension. However, when imagining the pipe walls as tiny rectangular plates, each with its own neutral axis, it appears that both tension and compression exist on one side of the overall neutral axis. This is a paradox, but it is explained by the fact that the stress and strain are not evenly distributed over the entire cross section. In real-life bending situations, factors such as material thickness, grain direction, and bending techniques can also affect the neutral axis and the overall bending process.
  • #1
mrmojorizing
7
0
Hi,

I'm a bit confused. Say you're bending a cylindrical pipe (tube, hollow cylinder). So the neutral axis of the pipe will be in the middle of the pipe and all the material on the inside of the bend (to one side of the neutral axis) will be in compression, while all the material on the outside of the bend (on the other side of the neutral axis) will be in tension.

Now imagine you have a rectangular steel plate (a rectangular steel slab) and you bend it. The neutral axis will be in the middle of the plate and all the material on one side of the neutral axis will be in compression, and on the other side will be in tension.

My question is this: when you're bending a cylindrical pipe you can imagine the pipe walls being composed of tiny rectangular plates, each of which has its own neutral axis. Each tiny rectangular plate if bent on its own would have compression on one side of the neutral axis and tension on the other. This would seem to indicate that on one side of the neutral axis (axis of the pipe not of a rectangular plate) there is both tension and compression, since the pipe wall can be imagined to be made up of tiny rectangular plates, each of which has its own neutral axis. Yet if you look at the pipe alone, without imagining that the walls are made up of tiny rectangular plates there is supposed to be only tension or compression on one side of a bent pipes neutral axis. So this is a paradox which i don't get. What am i doing wrong? See pic below if you don't get my question.

https://docs.google.com/file/d/0B8Ru4CVOjev0aUpsbVk2cGl3aTQ
 
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  • #2
Sorry, no pic attached.
 
  • #3
For a given cross section shape, there is only one neutral axis for the overall cross section. If you look at the little rectangles in the pipe, the stress and strain are indeed varying (but just a little) over the cross sections, but the averages are not zero. On the outside of the bend, they are all in tension, although the tensile stress does vary slightly with distance from the neutral axis (of the overall cross section), with portions of the rectangles further from the neutral axis having more stress and strain, and portions closer to the neutral axis having less stress and strain.

Think of bending a deck of cards. Why is it easier to bend an ordinary deck of cards than it would be if you glued all the cards together? This should give you a hint about your question.

Chet
 
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  • #4
mrmojorizing said:
Hi,

I'm a bit confused. Say you're bending a cylindrical pipe (tube, hollow cylinder). So the neutral axis of the pipe will be in the middle of the pipe and all the material on the inside of the bend (to one side of the neutral axis) will be in compression, while all the material on the outside of the bend (on the other side of the neutral axis) will be in tension.

Now imagine you have a rectangular steel plate (a rectangular steel slab) and you bend it. The neutral axis will be in the middle of the plate and all the material on one side of the neutral axis will be in compression, and on the other side will be in tension.

My question is this: when you're bending a cylindrical pipe you can imagine the pipe walls being composed of tiny rectangular plates, each of which has its own neutral axis. Each tiny rectangular plate if bent on its own would have compression on one side of the neutral axis and tension on the other. This would seem to indicate that on one side of the neutral axis (axis of the pipe not of a rectangular plate) there is both tension and compression, since the pipe wall can be imagined to be made up of tiny rectangular plates, each of which has its own neutral axis. Yet if you look at the pipe alone, without imagining that the walls are made up of tiny rectangular plates there is supposed to be only tension or compression on one side of a bent pipes neutral axis. So this is a paradox which i don't get. What am i doing wrong? See pic below if you don't get my question.

[PLAIN]https://docs.google.com/file/d/0B8Ru4CVOjev0aUpsbVk2cGl3aTQ[/QUOTE]

I can't see your pic though I'll say volumes have been written about bending metals.

Sheet steel bending allowances are usually based on something like a 44% of the material thickness as far as the actual neutral axis(inside radius). This is not something engineers came up with on paper, the brake operators had a lot of input in this. So abandon the idea of a 50% neutral axis because it does not work in real life with sheet. Bending with or against the grain also affects the force required.

With pipe and tubing it gets far more complicated. The material will flow giving you a bevel based on your bend radius, material, and length of the bent leg coming off the mandrel centerline.

My point is even with the best mechanical systems we have to bend pipe the bend axis will vary as the pipe itself is 1. captured 2.forced along the mandrels 3. Released off the mandrels.
 
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FAQ: Bending pipe - compression and tension

What is the difference between bending pipe through compression and tension?

Bending pipe through compression involves applying external forces to the pipe in order to create a bend, while bending through tension involves using internal forces within the pipe to create a bend.

What types of pipe materials can be bent through compression and tension?

Pipes made of materials such as steel, aluminum, copper, and plastic can be bent through compression and tension. The type of material and its thickness will determine the amount of force needed to bend the pipe.

What are the advantages of bending pipe through compression?

Bending pipe through compression allows for greater control over the angle and shape of the bend. It also requires less force and can be done using simple tools, making it a more cost-effective method.

What are the advantages of bending pipe through tension?

Bending pipe through tension can be done without damaging the exterior surface of the pipe, making it useful for pipes that require a smooth finish. It also does not require external tools, making it a more convenient method for bending small or delicate pipes.

What factors should be considered when choosing between compression and tension for bending pipe?

The type of pipe material, the desired angle and shape of the bend, and the available tools and equipment should all be considered when choosing between compression and tension for bending pipe. Additionally, the cost and time constraints of the project should also be taken into account.

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