Re: [Harp-L] Brass vs Stainless Steel

["Vern Smith" <jevern@xxxxxxx>]

----- Original Message ----- 
From: <jandkday@xxxxxxxxxxx>
To: <harp-l@xxxxxxxxxx>
Sent: Friday, September 05, 2008 6:32 AM
Subject: [Harp-L] Brass vs Stainless Steel

> ..... I always thought brass to be a softer metal,
> but does it's pliability make it easier to bend?  Stainless steel seems 
> stronger but is that a
> good thing in a harp?   Any engineers want to chime in?  I'm in over my 
> head.

Definitions:
"Stress" refers to the amount of force (per unit area) applied to a piece of 
metal.
"Strain" refers to the stretch (per unit of length) as it deforms.

When you stress a piece of metal, up to a point called the "elastic limit," 
then release it, it will return to its original shape.  Below the elastic 
limit,  strain is proportional to stress. The elasticity of a metal is the 
ratio of stress divided by strain. Steel has a higher elasticity than brass, 
i.e. it takes more force to get a given amount of bending. Some steels have 
a stress level (lower than the elastic limit) called the "fatigue limit". 
Below that stress, you can exercise the metal for an unlimited number of 
cycles without fatigue failure.

If you stress a metal beyond the elastic limit, it enters a plastic zone 
where strain is no longer proportional to stress and it will not return to 
its original shape.  Its behavior in thiis zone is described by its 
ductility.  Ductility is roughly the amount of strain it will take before it 
breaks. The opposite of ductility is brittleness. Brass is more ductile than 
steel.  Glass is an example of extremely low ductility and gold is an 
example of extremely high ductility.

When we change the gap of a reed, we bend it beyond its elastic limit so it 
will not return to its original shape but will assume the new gap. 
Beryllium copper is a very elastic spring material with high resistance to 
fatigue.  However, it is very brittle and easy to break when re-gapping.

Because of its higher elasticity, a SS reed will be thinner near the rivet 
where it bends than a brass reed of equivalent pitch and responsiveness. 
However, because SS is less dense than brass, it will be thicker at the tip 
than the equivalent brass reed.  If each is reed is designed to have the 
same pitch and responsiveness, there will be no perceptible difference in 
tone between reeds of different metals.

Metallurgists may chide me for omitting some exceptions and qualifications. 
The rest of you may chide me for getting too technical.

Vern