Best metal for cyclical stresses.

[Mudstud]

I have to design a pipe that will deliberately be subjected to cyclical stress, eg the pipe will have a motion similar to a skipping rope although the amplitude of vibration will be 0.1m and the pipe length will be about 30 m. The pipe will be used until it fails and then will be replaced, to keep costs down I want to use a pipe that will not fatigue and fail. The pipe will be under tension and pressure. What is the best metal or type of metal to use to minimise fatigue and fracturing. PS I'm not a metallugist so keep answers simple!

 

[metengr]

Mudstud;
First off, you need to know the fluid(s) the pipe will convey and pressure/temperature to select a suitable material, and optimize the ID to assure flow velocities are acceptable.

My advice to you is to obtain a copy of one of the pressure piping codes, like ASME B31.1 or 3 to understand what is involved with pipe design, in general. Even if the pipe is low pressure, you will be able to design the piping system using guidance from the pressure piping codes.

 

[CoryPad]

Medium carbon steel, quenched and tempered, final hardness ~ 35 HRC.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[arunmrao]

EN 24 grade steel Quench and tempered to 35-40RC

Chocolates,men,coffee: are somethings liked better rich!!
(noticed in a coffee shop)

 

[metengr]

Mudstud;
There are a wide range of possibilities here in materials and I should have probably asked what is the application since you mentioned the pipe is under pressure.

By the way, what exactly is your application? Is this for a structural purposes, to convey fluids, to build a large shock absorber, a pipe support, what....??????? You may need to consider welding spools together at 30 meters length, which factors into material selection....

 

[civilperson]

Copper nickel alloy, (Monel 500), with thick walls and large diameter can have cyclic stresses less than the fatigue limit for corrosive liquids at pressure. Thirty meter span with only 0.1 meter deflection will require a diameter of more than 3 meters if filled with water.

 

[Mudstud]

Thanks all - this gives me a starting point. In a couple years if/when this develops from hair brained idea to standard industry practice I'll have to remember to acknowledge this webpage.

 

[dik]

I don't know if it's applicable for your use... but, copper-beryllium (sp?) alloys can have a very high fatigue resistance. Often used for springs, etc.

Dik

 

[sreid]

As a very broad brush rule of thumb for steel, if you keep the maximum stress below 1/2 the yield stress, the fatigue life will be infinite. Fatigue failure occurs due to crack propogation so a smooth (ground) surface will be less prone to fatigue failure. Shot peening also helps by putting the surface in compression.

 

[davefitz]

The problem you described sounds like fluid-eleastic vibration of a pipe- that problem will have the deflection and stress related to the pipe's moment of inertia and the Young's modulus of elasticity. So, the problem cannot be solved just by naming a metal- the entire problem must be solved for a number of candidate metals and then you pick the winner.