To what degree does a hole in a tube make it weaker?

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How much is a circular pipe with holes in its walls weaker than a pipe without holes, specifically when subject to bending and torsional forces?

Say I have a hardened steel pipe of some diameter, length and width, and a hole (of max 1/5 the pipe diameter) is drilled horizontally, through both pipe walls, in the centre of the pipe.
The pipe would be subject to a reasonable degree of torsional force, a small degree of horizontal bending force, but a high degree of vertical bending force.
In this scenario, force is always applied along the whole length of the pipe and fractures in the metal are unacceptable.

As per the question title, to what degree would this hole make the pipe weaker?

Would it always severely weaken the pipe structure because there is a weak point along its length, or would it weaken it to a lesser degree; in relation to the hole size?
And additionally, in either case, if one hole made the pipe X amount weaker, would many more holes along the length (1 diameter apart and on the same axis) compromise the pipe structure any more?

Many thanks in advance!
 
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  • #2
jrmichler
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Short answer: Adding holes to a pipe will make it weaker. Possibly a little weaker, more likely a lot weaker.

Longer answer: In order to find out how much weaker, you need to do separate stress calculations for bending and torsion. If the loads are time varying, or the pipe is turning, then you may need to perform fatigue calculations. You need all dimensions and loads, and the steel properties in the as hardened condition. You need to define your reliability and/or safety factors. Then you can start the calculations.

This is a job that an experienced engineer can do in a couple of hours. It is beyond what is practical in this forum. And if the shaft breaking would cause safety issues, you definitely need an experienced engineer to perform a full analysis.
 
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  • #3
Baluncore
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Welcome to PF.

The pipe would be subject to a reasonable degree of torsional force, a small degree of horizontal bending force, but a high degree of vertical bending force.
We would need to know the initial orientation of the pipe axis. You need to make a sketch of the situation and attach it to this thread. Also describe the application and consider the safety implications.

Drilling only one hole will reduce strength and rigidity. Drilling more holes, if sufficiently far apart, will not further reduce the axial tensile strength, but it will increase the flexibility and lead to earlier failure in axial compression.

Holes are usually drilled horizontally through horizontal structural members on the mid-line or neutral axis so as to not reduce the beam flange dimensions. Different engineering codes specify different maximum permitted hole sizes and separation.
 
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I appreciate the replies, thank you both.

My question omitted any specific details in case there was a rule about stress, relating to holes, that I was missing. The details of what I was asking it for weren't really relevant, because it was just hypothetical.
I was looking at ways to make heavy trucks lighter, and I was thinking about a backbone (central tube) chassis for that. Ultimately I ended up resolving my own question anyway, which I probably should have figured out before asking, but anyway.

There's a vehicle manufacturer (Tatra) which uses backbone chassis' for their vehicles, including heavy trucks. They run the driveline through the chassis, and every axle is driven, so thus there is double the holes in the tube as there are axles... and they are not small holes of course.
The fact that there are backbone chassis' to start with answers my question, and furthermore, the dual C channel chassis rails that I was trying to improve upon are basically half-tubes that have hundreds of holes through them.

So to what degree does a tube weaken with holes? Not significantly; there is no Achilles Heel like I was wondering about – it is going to be weaker of course, but by how much depends on the details. The amount to which it does is logical, however.

Welcome to PF.
Thanks! It seems like a great place to get answers.
 
  • #5
Baluncore
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, and furthermore, the dual C channel chassis rails that I was trying to improve upon are basically half-tubes that have hundreds of holes through them.
The advantage of dual channel rails is that they will twist along the truck which reduces the stress when the truck crosses rough ground. If a tubular backbone chassis is used that has reduced flexibility, it will put much higher loads on the chassis and suspension components. The lowest weight chassis will be flexible and will not have a heavy spine or torque tube along the neutral axis.

Dual channel rails also better support the sides of the load and so reduce axial torque and sway. For example, when vehicles are loaded on a tray truck the wheels bear down directly onto the two channel rails.

Many trucks have a second channel section wrapped over the first. That increases local strength while retaining chassis flexibility. That double plating is only used where major loads are applied to the chassis channels.

So to what degree does a tube weaken with holes? Not significantly; there is no Achilles Heel like I was wondering about – it is going to be weaker of course, but by how much depends on the details.
The tubular chassis must be reinforced where holes give access for drive shafts and where the wheel supports must be attached. If that is not done, the total weight of the tube will be significantly increased.
 

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