Why do corrugations give strength to pipes?

I have read that corrugations give strength to pipes and the same strength can be achieved with thinner wall when compared to plain pipe.

But I don't understand why (how) the corrugations gave strength to the pipe. How do they work?

For a thin wall structure, corrugations behave like a frame, the outer shell rests upon. Imagine the fuselage of an airplane or the hull of a ship. They are typically consist of a strong framing, made by bars, both in the cross-sectional plane and longitudinal ones.

The cross-sectional bars (the circular rings in the image above) provide stiffness and resistance against the normal forces towards the fuselage (either due to internal or external high pressure), while the longitudinal bars support the rings against lateral buckling and provide longitudinal stiffness.

Without such framing, a shell structure should rely on it's own thickness to resist normal wall pressure (through bending) as well as lontidutinal forces. Not an efficient use of material.

Edit: For pipes, internal corrugations are typically only longitudinal ones, since otherwise flow would be severly inhibited. In that case, the resitance against normal wall pressure isn't as much improved by the longitudinal corrugations alone, but they provide an improvement for local phenomena, and for short pipes, or for pipes with external reinforcement rings. In such cases they function like beams for the thin shell.

The increase in strength in terms of moment of interia is small since the change in overall size is small. This also doesn't address the different properties of the metal when it goes through the corrugated process. When the corrugated metal is made it is cold-worked which takes that portion of the metal beyond its yield point. This makes the bent portion of the metal stronger and more force is required to reach its new yield point. So, when the corrugated sheet is made, it is the bending process that creates a new geometry that adds strength but you're also changing the material property of the metal making it stronger.

For more information on the cold-working principle look at stress-strain curves if you want to read more about it. It's also called strain hardening.

The corrugations will increase the moment of inertia of the pipe, making it more resistant to deformation out of its circular shape.

Imagine you slit the pipe long ways and unroll it into a corrugated sheet. It is easy to understand that the corrugations will make the sheet more rigid and resistant to bending along that direction. The same principle will apply to the pipe.

• Bending has nothing to do with moment of intertia. Corrugation works becaus an attempt to bend in one direction is resisted due to the 'magnified' bend requirements on folded sections. – Carl Witthoft Mar 10 '17 at 16:16
• Second moment of area is the more technically correct term, but it is commonly referred to as moment of inertia. It most definitely has to do with bending. – S Barry Mar 15 '17 at 21:51

Think of an egg. It is able to withstand substantial force die to its shape longitudinally yet laterally it is weak. The same idea applies to corrugation. The perpendicular forces exerted on a corrugated pipe are transformed into lateral forces due to the shape of the configuration of the corrugation. The different depth at which the compressive forces act on the pipe create tensile and compressive forces in the pipe material which means that the pipe wall can be thinner and therefore lighter for transportation.

Each peak resists compressive forces whilst each valley resists with tensile strength radially. This is why damage to the valley section in a corrugated pipe is more likely than at a peak and radial damage is far more likely than lateral damage. Once the pipe is buried, the tensile forces also balance out to become compressive forces due to the covering material being the opposing yet equal force acting on the structure.