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

["Jonathan Metts" <jonathan@xxxxxxxxxxxxxxxxxx>]

Oh dear, you're exactly right.  That's what I get for trusting my memory! 
Yes, the elastic region means no permanent deformation, while the plastic 
region does have permanent deformation.  I'll remember the term "stress 
cycling" and bounce that off my professor.  He is primarily an alloy 
materials guy, so he should know plenty about this stuff, but perhaps not as 
much as someone with an email address at fea.net!  ...assuming that fea is 
the same as the FEA we've been talking about.

Jonathan Metts

----- Original Message ----- 
From: "Vern Smith" <jevern@xxxxxxx>
To: "harp-l" <harp-l@xxxxxxxxxx>; "Jonathan Metts" 
<jonathan@xxxxxxxxxxxxxxxxxx>
Sent: Tuesday, August 16, 2005 12:10 AM
Subject: Re: [Harp-L] Re: Breaking in Marine Bands


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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