Re: [Harp-L] Re: Breaking in Marine Bands

["Vern Smith" <jevern@xxxxxxx>]

Your analysis below will be correct if you substitute "the elastic region" 
for the "plastic deformation region".  In the elastic region there is no 
permanent deformation.  In the plastic region, there is. Although corrosion 
can accelerate fatigue,  long periods of stress cycling can fatigue copper 
alloys even when no corrosion is present.

Vern
Visit my harmonica website: http://www.Hands-Free-Chromatic.7p.com


----- Original Message ----- 
From: "Jonathan Metts" <jonathan@xxxxxxxxxxxxxxxxxx>
To: <moorcot@xxxxxxx>; <harp-l@xxxxxxxxxx>
Sent: Monday, August 15, 2005 4:38 PM
Subject: [Harp-L] Re: Breaking in Marine Bands


> Is there a physicist in the house?  ;-)
>
> Steve

More importantly, is there a structural engineer in the house?  I'm not one, 
but I have taken some classes in the field and have access to a couple of 
professors who can probably answer this question.  From what I remember in 
my Structures classes, metals generally do not experience degradation from 
repeated loadings as long as those loadings remain within the "plastic 
deformation region", that is, the loads are not so great that they 
permanently deform the metal.  When you regap a reed, you are causing 
non-plastic deformation because the reed's shape is permanently changed. 
When you blow or draw across the reed, you are causing plastic deformation 
because the reed's shape goes back to its previous state once you stop the 
airflow (load).  This all goes for aluminum alloys at least, because those 
were the focus of my coursework, but I'm pretty sure the concepts work for 
brass as well.  My professor will be able to confirm.

I remember be astonished in that class that there was no mention of 
long-term changes in the metal even when the loads are always in the plastic 
region.  So I asked my professor at the time, and he said there is a 
phenomenon known as "metal fatigue" that is related to the frequency of 
loading.  However, metal only fatigues under certain temperature and 
humidity conditions, usually underwater.  It is certainly possible that the 
humidity of our breath puts the brass reeds in such a wet environment that 
the reeds can fatigue over time.  As I said, I'm going to ask some people 
smarter than me this week and get a more confident answer for us all.

Jonathan Metts

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