Reed Fatigue

[Dick Anderson <anderson@xxxxxxxxxxxxxx>]

I have long been interested in the forces acting on reeds and damage caused
by the various legendary taboos such as:  Not breaking in a reed, bending
on 5 draw, bending too loudly.  I even read one report of reeds "healing"
themselves by aging under one guy's bed.

The first mechanical engineer I talked to mentioned the work hardening
theory of reed destruction, but I discounted that theory on the same basis
as others have mentioned that if the reed was getting stiffer, the note
produced would go up.  Instead, we notice reeds going "flat" or lower as
they age and wear out.

Also, I think work hardening only happens when a metal is stressed beyond 
it's elastic limits causing a permanent deformation.  An example of bending a
coat hanger wire was given earlier, but I'm sure the writer was speaking of
permanent deformation, not an elastic cyclical motion of the wire.

If springs work hardened under elastic cycling, then automobile 
coil springs which suffer small elastic deformations all their life should 
get stiffer and stiffer until finally they crack and break.  This is also
not seen in practice.  Although I have seen leaf springs crack and break.

I have been reading about metal fatigue and am in quite over my head, but
from the texts I have been able to glean these few gems:


1.  Cyclic stresses below the point of permanent deformation do cause shifting
    of the atomic structure and eventually lead to crack formation and growth
    which eventually leads to complete failure. (Reed broken off)  

2.  The larger the deformation and the higher the number of cycles, the sooner
    that a crack will develop and grow.

3.  Cracks always begin at the surface.

4.  Crack formation can be reduced by stress relieving the surface of the
    metal.  (Shot peening is used to improve life of steel springs)

5.  Crack formation can be reduced by improving the purity of the metal
    avoiding inclusions and defects built into the material.


Bottom line:  Reeds fail due to crack formation as the result of cyclical
              elastic deformation over time.  This is a classic metal fatigue
	      problem.

My Hunches:   Bending notes on the harmonica causes the "opposite" reed to
	      vibrate outside of the slot in an "opening" manner.  This may
	      cause greater stress on the reed since its vibrations could
	      be of higher amplitude than a reed vibrating in a "closing"
	      manner. (Johno, have you tested this theory?)

	      Harmonica manufacturers could improve the longevity of reeds
	      by stress relieveing the surface perhaps by micro-peening or 
	      smoothing sharp edges left by grinding the reeds into tune. 
	      (Hohner reeds have lots of small scratches left by the tuning 
	      process while Suzuki "laser tuned" reeds are somewhat smoother)

Old wisdom:   I spoke with an organ reed maker who said that the reeds in some 
	      organs have lasted more than 100 years with consistent use.
	      He did warn me that the key to long life was to control the
	      air pressure to the reed and NEVER to blow into a reed to
	      test it since the pressure was not controlled.  (Organ reeds
	      are "beating reeds" and not "free reeds" like a harmonica,
	      but the movement of the reed is still cyclical and elastic.)
	      

My question to Johno or others:

             Has any quantitative work been done to correlate reed life with:

	      A. Loudness of playing
	      B. Amount of bending
	      C. Lack of "break in" 
	      
             Has any similar work been done to correlate guitar string 
	     life with the factors above?

Love to talk harps!

dick....
	     
--

Dick Anderson  CCMO New Product Engineering    Telnet                 229-3110
Hewlett Packard                                Direct Dial      1-303-229-3110
3404 E Harmony Road                            HPDESK     dick_anderson@hp4000
Fort Collins Colorado 80525                    mail  anderson@xxxxxxxxx