Stainless steel for fatigue (vibrations) resistance ? 2

[ratatax]

Hello,

I need to choose a material for the sound rods i use in my musical instruments. There will be thin (3mm dia) metal rods, clamped at one end, that vibrate to produce sound when striked. I first chose Brass for its acoustic qualities (seems to resonates longer than steel for the high harmonics), but after reading some documents about metal fatigue i'm not sure about the Brass choice anymore.
Since i try my best to build my instruments with the highest quality, i'd like it lasts a long time before it needs repair.

Vibration frequencies : from 10 to 10000 Hz (the lowest vibrations are of big amplitude - still < 30% elastic limit -, the highest are of very small amplitude)


What is the best material for me ?

I have easily access to CuZn39Pb2 or CuZn36 brass / 304 or 302 stainless steel.

I've heared that low carbon steels (typically stainless steels) are good for fatigue purposes ?

 

[EdStainless]

The problem is with the resonance. Brass has a much lower modulus and you will get different harmonics.
Making the part in stainless steel will require very different dimension, and it will not produce as warm of a sound. I will be missing the fullness of sound.
The leaded material will sound the most interesting (they use high lead brass for organ tubes) but it has the worst mechanical properties.
If you go with stainless make sure that it has been cold finished to a high strength. This has no impact on the sound quality, but it will give you very good fatigue properties and make them harder to damage. The tensile strength should be above 120ksi (850MPa).

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P.E. Metallurgy, Plymouth Tube

 

[ratatax]

Thank you for your answer, i'll ask my supplier for the strength of their stainless steel

Do you know what affects sound quality in a material ? Lower modulus means more sustained harmonics ?

 

[EdStainless]

For the same size the lower modulus material will have a lower resonant frequency.
I believe that the higher modulus material has lower energy in the harmonics (3rd, 5th and so on).

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P.E. Metallurgy, Plymouth Tube

 

[Tmoose]

Are you concerned with fatigue because devices like this are known to fail sometimes?

The detail of how the bar is clamped will have a profound effect on the stress there, which is likely the location of highest stress.

http://iberisa.files.wordpress.com/2013/11/cantilever_beam_nodal_stress.png

Poorly chosen or executed details can increase localized stress more than 3 times.

If the maximum bending tip amplitude is small and the resulting stress is low enough then fatigue will not likely be a problem with either material or any mounting detail .

Poor Geometry can defeat even fancy materials every time.

 

[EdStainless]

Moose has a valid comment, design is critical.
If you make the end that mounts slightly larger, and then use a gentle radius to blend down to your desired diameter you can clamp the larger section and greatly reduce stresses at that location.

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P.E. Metallurgy, Plymouth Tube

 

[ratatax]

I have no luck.. the problem is I need to do a tapering at the clamped end, to make the rod in-tune with itself (harmonic tuning) :

You can see the same design used for toy pianos : (this pic is not mine)

my instruments are sort of improved version of the toy piano

I plan to use felt with punched holes, put around each rod, to prevent any excessive vibration

Since the acoustics forces me to use a very bad stress/fatigue design, i guess I have no other solutions than using the best material and striking it as gently as possible ?

 

[Tmoose]

Just because they taper from large to small near the clamp, it does not mean the transition back to large for clamping doesn't matter. I bet there are "good" and "bad" ways to do it.

 

[ratatax]

You're right.

I have no experience dealing with fatigue, but i guess i should grind the rods to a shape like pic #3, to avoid concentrating too much stress on a small area ? + Polishing should help to prevent crack initiation

 

[tbuelna]

If fatigue is a concern with your cantilevered rods, whether you use brass or stainless steel anything you can do to improve performance for reverse bending at the clamped end will help. Drawing, rolling, peening, cold working, polishing, etc. of material in this surface area will improve fatigue life. Some type of corrosion protection coating would also help if brass rods are used.

 

[EdStainless]

Ratatax,
Grind the transitions smoothly, no steps or notches.
In your photo see how the ground area is more rusted? They left it very rough.
It needs to be at least as smooth as the rest of the rod.
Rough grind them and then smooth with sandpaper until you can't see the grind marks.
This will help a lot.
With brass you will have larger rods, and get better tone.
But make sure that you coat them to resist corrosion. A polyurethane would do.

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P.E. Metallurgy, Plymouth Tube

 

[Frederick]

Treat the tubes with cryogenics. It will increase the fatigue life considerably and has been found to improve the sound and playing characteristics. Yamaha offers cryo on some of their instruments, so it has been tested extensively.

 

[EdStainless]

Cryo has no impact on an austenitic stainless.
Steels, yes, but not stainless.

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P.E. Metallurgy, Plymouth Tube

 

[dgallup]

I'm still not convinced that these are particularly highly stressed. Have you ever seen one break?

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[EdStainless]

I have seen similar percussion devices, and they broke when the tuning grind was very rough and abrupt.
I have a daughter who is a percussionist, I have fixed a lot of things over the years.

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P.E. Metallurgy, Plymouth Tube

 

[ratatax]

dgallup : Yes i've broken some rods but it was my bad for this prototype : bad material + bad design : aluminium, rough grinding and no smooth transition at the clamped end. The first note broke after 3 months of non-intensive use, the others are still working but they're probably all slowly diying.

The rods only get a high stress when the key is pressed (hammer strike), the sustained vibrations are of less amplitude and decays in 0.5 to 10 seconds depending on the note.

EdStainless : Do you know what instrument it was ? I'm curious.

 

[EdStainless]

I have no idea who built the original. It was played with mallets and sounded like a vibraphone

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P.E. Metallurgy, Plymouth Tube