Elastic limits,

[A440A@aol.com]

Greetings all,

I read in Jim Coleman's (<snipped>) response;

 > using strain gauges to mark what
>happens when a string breaks. When tension is gradually added to a string
>without frictional resistances (such as agraffes) a string begins to
>yield a bit before it breaks.  This is where time enters the picture.
>
>Here are some examples of tests taken at the CGConn engineering labs while
>I was there.
>
>wire size    yield point    break point
>
>13           258 lbs        290
>13.5         267            303
>14           317            355
>15
>16           347            398
>18           431            487
>
>
>From all of this you can see where time enters into the picture. There is
>a measureable period of time between when the string comes to the yield
>point and the point where it breaks.  My impact method obviously
>allows me to live somewhere between the yield point and the break point,
>because I definitely do have less string breakage now than I used to have.

<snip>
>There can be as much as 10 to 20 pounds greater tension in the segment
>between the tuning pin and agraffe as there is in the speaking length
>while pulling pitch up.

      I have to ask,  if these numbers are giving the yield and breaking
tensions of music wire,  and the usual string tension in a piano is a
percentage of the yield, ( 80%, approx?),  then how are these higher  numbers
applicable to our tuning at the much lower tensions pianos use?  Anybody got
scales that use music wire at  250 lbs for size 13??  It seems not likely.
  Is the capo, or agraffe friction able to generate the difference in tension
between speaking length and tuning pin?

>the Chickering (which I did this morning).  The string approaches the
>tuning pin at such a high angle from the V-bar Agraffe pin that it tends
>to climb over the previous coil on the pin.  A brand new string can break
>easily at that point of greater bend. Before raising the pitch of this
>piano, I took the precaution of changing the angle of the coils on the
>pin and tapping the string back down under the previous coil.

     I would suggest,  that breakage is usually caused by the elastic limit
being  well surpassed on the outside of the curve of the wire, where it goes
around the agraffe, or V-bar.  This outer circumference  is the place where
breaks begin.  If the string is put near it's break limit, and then forced to
bend,  the wire will micro-fracture on the outside bend.  If these fractures
continue, they effectively reduce the diameter of the string at this site,
and the breaking tension for this site is reduced.  Eventually,  that limit
will decrease to the tension used in play, or tuning, and the string will
break at it's weakest section.   I believe that this is the reason for the
propensity of steeply angled strings to break, rather than the excessive
friction generated by the sharpness of the turn.
     But perhaps I am totally missing something here,   ( I have found a
forest before by walking into a tree......)

Regards,
Ed Foote
Precision Piano Works
Nashville, Tn.