Specification of brass needed for replacing old Lachenal reed tongues

[Richard Mellish]

I find it reassuring that the vibration of a reed in use doesn't cause work hardening. It it did, that would presumably raise the Young's Modulus and therefore the pitch.

 

Metal fatigue seems very plausible. Might it start from microscopic flaws in the edges of a reed, left from when it was cut to size?

[ttonon]

Richard, It's a common misconception that work hardening a metal increases its Young's Modulus.  It looks simply like, "Well, steel has a higher Young's Modulus than does brass and steel is harder than brass, so I guess if you make the hardness of brass up closer to that of steel, you will also make its Young's Modulus up closer to that of steel." 

The misconception arises because we are talking about two different mechanisms within the metal.  One is elasticity and the other is plasticity. 

Young's Modulus is a feature of elasticity, and elasticity is possible only for small enough stresses.  Such sufficiently small stresses involve only the intermolecular forces within the material.  You can visualize them as tiny springs.  That implies that only the chemical nature of the material participates in the observed elastic property. 

Once you stress the material so much as to cause plastic deformation, you involve other forces than just the intermolecular forces.  Every metal has internal features that tend to resist deformation, such as grain boundaries, inclusions, and dislocations. With plastic movement, these features get "locked in" more than they were just before deformation, and subsequent deformation escalates the resistance to relative movement that these features provide.  That explains the hardening, why it's harder to cause more deformation. 

But the intermolecular state is not changed appreciably by the deformation.  That's probably because the total volume of the material occupied by the grain boundaries and other defects is very small compared to the total volume of the material.  Thus, if the material is only slightly stressed again after the deformation, those tiny springs behave just as they did before deformation. 

[Richard Mellish]

This is

3 minutes ago, ttonon said:

Richard, It's a common misconception that work hardening a metal increases its Young's Modulus.  It looks simply like, "Well, steel has a higher Young's Modulus than does brass and steel is harder than brass, so I guess if you make the hardness of brass up closer to that of steel, you will also make its Young's Modulus up closer to that of steel." 

The misconception arises because we are talking about two different mechanisms within the metal.  One is elasticity and the other is plasticity. 

Young's Modulus is a feature of elasticity, and elasticity is possible only for small enough stresses.  Such sufficiently small stresses involve only the intermolecular forces within the material.  You can visualize them as tiny springs.  That implies that only the chemical nature of the material participates in the observed elastic property. 

Once you stress the material so much as to cause plastic deformation, you involve other forces than just the intermolecular forces.  Every metal has internal features that tend to resist deformation, such as grain boundaries, inclusions, and dislocations. With plastic movement, these features get "locked in" more than they were just before deformation, and subsequent deformation escalates the resistance to relative movement that these features provide.  That explains the hardening, why it's harder to cause more deformation. 

But the intermolecular state is not changed appreciably by the deformation.  That's probably because the total volume of the material occupied by the grain boundaries and other defects is very small compared to the total volume of the material.  Thus, if the material is only slightly stressed again after the deformation, those tiny springs behave just as they did before deformation. 

This is straying somewhat from the subject of the thread, but my presumption was that any changes within the metal that affect its mechanical properties would have some effect on the elasticity. If I understand you aright, the "hardness" that is increased is relevant only to plastic deformation, not elastic.

[Richard Mellish]

Posting this separately, as it is a bit closer to the subject of this thread.

 

A link from another thread took me to http://www.concertina.com/worrall/beginnings-concertina-in-ireland/index.htm, where there is a copy of a Joseph Scates advert, which mentions "Gold Notes, which never require tuning, and cannot be broken".

 

Allowing for the exaggeration in an advert from before the days of advertising standards, what might those "notes" have been made of that would have made them at least somewhat more durable than the usual ones?

[ttonon]

11 hours ago, Richard Mellish said:

If I understand you aright, the "hardness" that is increased is relevant only to plastic deformation, not elastic.

Correct.

 

Tom