STME58
624 posts
Jan 04, 2014
12:28 AM
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It will be a long time, if ever, before we see this in a harmonica reed, but this is a fascinating advance in the science of fatigue (the main failure mode for a harp reed).
MIT Metalurgy Advance
Last Edited by STME58 on Jan 04, 2014 12:33 AM
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Stevelegh
903 posts
Jan 04, 2014
1:23 AM
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I had a chat with Anthony Dannecker about this once.
You know when you gap a harp and go back to it a few hours later? It plays a little differently.
Apparently this is what its about.
OK, so which customiser is going to set up a video microscope and do a time lapse movie of the process?
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nacoran
7468 posts
Jan 04, 2014
10:48 AM
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If I'm looking at that image right, you'd have to pull the reed from the free end with a steady gentle force?
I've wondered if heating reeds would work. Get them just 'a little' melty without distorting their shape and see if that fixes the cracks.
There are some new age plastics that have resin beads embedded in them. When the surface is scratched they rupture and flow into the scratch and repair it. I'm not sure I'd want resin oozing anywhere on a harmonica though!
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Nate
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STME58
625 posts
Jan 04, 2014
11:25 AM
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There is a process called age hardening that occurs in non-ferrous metals. Alloys are solid solutions and the solute can come out of solution, even in the solid state. Some metals continue to get stronger for weeks or months. I m not an expert in this but I expect this may have something to do with the phenomenon @Stevelegh mentioned. The process is temperature dependent so @nacroran's idea of heating the harp to change the properties is not so far fetched. You don't heat it to the point where it gets "melty" you just add enough energy so that the grains boundaries can move more easily. There are charts available to determine what temperature and for how long to hold a metal to achieve maximum strength.
I have wondered if letting a harp rest for a week or two may allow a bit of change in the grain structure of the reeds.
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Greg Heumann
2545 posts
Jan 04, 2014
10:45 PM
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Google "metal hardening", "metal annealing", "metal normalizing", "metal tempering" "metal work hardening"- these are ALL different terms that have to do with re-arranging the molecules in metal. The temperatures involved can be in the hundreds or thousands of degrees.These temperatures are critical and must be precisely controlled. The rate of heating and cooling must ALSO be precisely controlled.
While you're at it, google "elastic" vs "plastic" deformation. And "metal fatigue".
If you bend a piece of metal back and forth, it work hardens. Sooner or later it gets hard enough that the same amount of bending cracks the metal. (You have all proved this to yourself with a paper clip.) If you bend it within its elastic limit, it returns to its original shape and position and lasts a certain period of time - a long time typically. If you bend it into its plastic range, it does not return to its original shape and position, and repeated bending causes it to fail much sooner. Some of the above-mentioned heat treating techniques may indeed relieve internal stress, but if you've gone into the plastic deformation, I'm not sure. And that's just what we do when we bend too hard, OR gap our harps.
If you don't understand the terms above you're not even close to understanding the problem. If you DO understand them but don't know how to they apply to the specific alloy of metal in your harp reeds, (where I'm at) you're still not in a position to speculate.
This is a subject for trained metallurgists. I've always wondered......
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Last Edited by Greg Heumann on Jan 05, 2014 9:29 AM
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STME58
626 posts
Jan 04, 2014
11:42 PM
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Greg,
I agree with what you said here, especially the part about this being a subject for trained metallurgists.
You left out age hardening, also known as precipitation hardening. This can occur at low temperatures on some alloys. I have some texts I will dig out that show the cooling curves required to get the various tempers for various alloys. As I recall, very small changes in the chemical composition of an alloy can have a great effect on the cooling rate vs grain structure curves. It has been a lot of years since I studied this as an undergraduate engineering student and I have not worked with it since. The grain growth involved in tempering or precipitation hardening is quite different that what was just discovered by MIT. They are seeing the structure heal under strain.
Finding the exact chemical composition of the alloy of a harp reed would be quite an undertaking. I would not be surprised if even the harmonica manufacturers did not have this information.
Last Edited by STME58 on Jan 04, 2014 11:45 PM
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