Posted by Rick Denney on September 08, 2003 at 10:14:17:
In Reply to: Re: Re: How many... Bell wrinlkes posted by Tom C. on September 05, 2003 at 17:32:46:
Huh?
The problem with work hardening is that people think it makes the material stiffer. It doesn't. Thus, work hardening cannot cause the changes in acoustical properties that people attribute to it. Those properties are only controlled by mass and stiffness, not by strength.
Work hardening really should be called work strengthening. When a material yields (i.e. when it is stressed beyond the point where it will spring back), the grains of the metal are aligned, which makes it stronger. The same thing happens when it is hammered, forged, cold-rolled, or any of a number of processes collectively called "working". The material can also be strengthening by heat treatment, where the heat is enough so that when it is cooled, the grains are pulled into alignment, but not so much that the metal runs together, which destroys that grain alignment. Heating to the point where grain alignment runs together is called "annealing" and it makes the material "dead soft." Again, it is just as stiff as before, but it is weak.
A spring that is dead soft has the same springiness as a hardened spring of the same dimensions, but it cannot carry the same load without yielding. On the other hand, hardened metal is more brittle, such that the yield strength is much closer to the fracture strength, which makes it much easier to propagate cracks.
This is why annealed brass dents so easily--the brass isn't as strong and will lose its ability to spring back at a much lower stress.
But when you bend the metal several times, as when you wrinkle the bell and again when you roll out the wrinkles, you work it, which makes it stronger and more brittle. Thus, with each succeeding damage/repair cycle, it takes more force to roll out the wrinkle, which brings the metal closer to the point where it will fracture (i.e. crack) under the stress of further damage and repair. How many cycles can a given bell endure? That is nearly impossible to predict because of all the variables that can affect it.
The only solution is to anneal the bell after a repair. Annealing requires very high temperatures--enough to burn off all the solder and certainly all the finish--and leaves the bell dead soft, making it susceptible to further damage. Thus, repair technicians are extremely reluctant to anneal a bell, though they might spot anneal around a cracked area to make it easier to repair.
Rick "who never claimed work hardening is a myth" Denney