Work hardening or "cold work" only occurs when a material is plastically deformed. This is basically when you bend a material and it doesn't return to its original shape. For example, you can bend the end of a paper clip a little and it will return to its original position. This is "elastic" deformation and will occur over and over as long as you don't go past the yield point. But if you pull it too far, it will plastically deform and return to a different, bent position. This is when work hardening will occur, since the grain structure of the metal has been altered. In normal playing/pedaling this should not occur. The excursion of the string is very small and should never exceed the yield point. Now, let's talk about fatigue. Fatigue occurs when a material is flexed over and over in it's elastic region. So, even though the end of the paper clip continues to return to its original position every time, if you do this, say, 100 million times it could eventually break due to fatigue. The number of cycles that a material can stand depends on how close to its elastic limit (where plastic deformation occurs) you get. The closer you get, the fewer cycles it takes for failure. As you deflect less, you get more and more potential cycles before an object breaks. There is a point (called the "endurance limit") below which the material will *never* break, no matter how many cycles it undergoes. It's possible that the piano strings undergo fatigue from vibration (lots of cycles), but the amplitude of the string is so small that I would guess that it's actually beyond the endurance limit, otherwise the most played strings (middle registers) would break the most. What is much more likely to be the culprit is the tuning process itself (and subsequent stretching of the strings). If you take a string out of a piano you can see where the plastic deformation is occurring, viz. at the bridge, agraffes, hitch pin, tuning pin and string rests. This is where you see permanent bends in the string. Every time you tune the piano you drag the string through this region, especially at the agraffe/capo. When you pull a string to a new bearing point (where the agraffe or bridge contacts the string) you are both bending a formerly straight segment *and* straightening a formerly bent segment of string. And the bend is "plastic" so you're work hardening the metal each time. Hope this helps! Don A. Gilmore Mechanical Engineer Kansas City ----- Original Message ----- From: "MKurta" <mkurta@adelphia.net> To: "Pianotech" <pianotech@ptg.org> Sent: Monday, February 09, 2004 5:15 PM Subject: Re: String Breakage > Hi Valentino: > Watch the pianist's right foot. Is it holding down the sustain pedal > all the time? This may create so much string movement that the wire "work > hardens" that is, it flexes so much that it becomes brittle and eventually > breaks. > Mike Kurta > > > _______________________________________________ > pianotech list info: https://www.moypiano.com/resources/#archives >
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