[Harp-L] Re: Steel Reeds

[Orjan Hansson <harpoh@xxxxxx>]

Hi,

I'm a long-time subscriber to harp-l (since 1998) and enjoy very much 
the discussions here, although I don't post very often. However, this 
issue with stainless-steel reeds interests me very much.

Here is my experience: I bought a Seydel 1847 in G a year ago and after 
half a year of (much) usage the 4-draw reed (A4) became badly out of 
tune - almost down to G4. I tried to tune it without success. In fact, 
close examination revealed a tiny crack ca 4 mm from the rivet. I 
contacted Seydel and they kindly offered to repair the harmonica. So two 
weeks after sending it, I had it back with the reed replaced. That's 
great service!

Now to my thoughts: Obviously, SS-reeds are prone to metal fatigue just 
as brass reeds. As I understand it, this can happen to any metal 
component that is subjected to a high number of deflections. This is 
very well explained in the article "Beyond the Basics - Performance Over 
Time: Fatigue Strength" that can be found here (I think someone posted 
this some time ago on the list but I don't remember who and when):

http://www.copper.org/applications/industrial/DesignGuide/homepage.html

Here is a quote from the article:

"Fatigue occurs because micro cracks develop on the metals surface when 
it is cyclically stressed. With repeated bending these cracks propagate 
through the metal thickness to a point where the remaining sound 
structure fails by ordinary rupture (because the load can no longer be 
supported)."

There is also a diagram which shows the relation between stress and 
number of cycles until fatigue sets in. The stress in turn is related to 
the deflection, which in the case of a harmonica reed depends on the 
blowing pressure. From the diagram one can conclude that a reed can last 
for a very large number of cycles if the stress (blowing pressure) is 
small. But if you increase the stress, the lifetime (number of cycles 
before metal fatigue sets in) can become drastically lower.

The diagram in the article is specifically for a certain brass alloy. 
But I guess that similar behavior can be expected for any metal. It is 
only a matter of *how many cycles* the component can last for a certain 
stress and geometry. Is there any metallurgist on the list that can 
verify this? In fact it would be very interesting to see a comparison 
between brass and stainless steel (with compositions similar to 
harmonica reeds).

Maybe someone with appropriate equipment can do an AB test? It would 
require blowing on reeds with a constant pressure until fatigue sets in, 
that is, when the pitch drops drastically. This might take several days. 
(100 million cycles of an A4 reed will take approx 63 hours!-)

By the way (as a comment to smo-joe), the thickness of the SS-reeds in 
the Seydel 1847 seems to be comparable to ordinary brass reeds (judging 
by eye - I haven't measured exactly).

Regards, Orjan
http://ohw.se

> On Apr 15, 2008, at 11:11 PM, lil Buddha wrote:
>
> > I am not a metallurgist, but from what I have read, steel does not  
> > share the
> > same fatigue issues brass does.
> > Many metals will eventually fail even with the smallest loads; the  
> > effects
> > of the stress applied are cumulative. No so with steel. As long as the
> > stress levels are kept below a certain level, the steel will never  
> > break.
> > Theoretically at least. This would indicate that those who have caused
> > failure in the stainless steel reeds are blowing harder than  necessary,
> > attaining a failure level instantly rather than cumulatively.  
> > Anything can
> > be broken. Failure of an object may be more the fault of the user  in 
> > these
> > cases.
> >
> > Forgive me if this was addressed and I missed it.
> > _

To which smo-joe replied:

> A reed must be made from anything spring. Steel is better in this
> regard, However, given the immense strength (by comparison) of steel,
> it stands to reason that the reeds would have to be MUCH thinner.
> Since there is so little material involved, ANY flaw will contribute
> to a failure.
>
> In an accordion, the reeds are huge (by comparison), and thus there
> is less chance of failure. That, AND they have a pressure relief valve.
> I agree with your synopsis
>
> smo-joe