stretching wire

[Richard Brekne]

Hi Dean...

Nice stuff here :)  after trying to sort through some of the er... heat
I stumble across your level headed post and it is yielding some good
reading. Thanks much for the positive, dispassionate and aparently
unbiased input.  I think I'll have a couple questions when I get time to
read through this thread more closely... but I'll shoot them to you off
line since by then this subject will no doubt have passed into the anals
of pianotech history... :)

Cheers
RicB


    I've done a little more research and found some numbers.

     >From Wikipedia:
    http://en.wikipedia.org/wiki/Tensile_strength#cite_note-0

    For most metals yield point is not sharply defined. Below the yield
    strength
    all deformation is recoverable, and the material will return to its
    initial
    shape when the load is removed. For stresses above the yield point the
    deformation is not recoverable, and the material will not return to its
    initial shape. This unrecoverable deformation is known as plastic
    deformation. For many applications plastic deformation is
    unacceptable, and
    the yield strength is used as the design limitation.

    This substantially concurs with what Mike says below, permanent stretch
    occurs when you pull the tension above the yield point. It is,
    however, a
    little misleading when it says "all" deformation is recoverable.

    I haven't yet been able to substantiate Mike's claim that with
    modern piano
    wire the yield strength is closer to the ultimate. I have found
    sources that
    place it at 43-35%. The Wiki article above is one and here is another:

    http://www.mech.uwa.edu.au/DANotes/springs/fatigue/fatigue.html

    This means if your design tension is above 45% of breaking strength
    you will
    exceed the Yield Strength, unless my sources are wrong. This is not
    really a
    problem as we are still a long ways from breaking strength.

    It is important to understand though that some plastic deformation does
    occur before Yield Strength, though not really appreciable. If
    you're really
    bored check out this site: http://books.google.com/books?id=dVQOAAAAYAAJ
 
<http://books.google.com/books?id=dVQOAAAAYAAJ&pg=RA7-PA319#PRA7-PA335,M1>
    &pg=RA7-PA319#PRA7-PA335,M1

    Scroll down to page 336, for example. You will find a strain table
    for 17
    gauge wire. At 100 lbs tension the wire elongated .0122 inches. When
    they
    took the 100 lbs off it had a permanent set (i.e., plastic
    deformation) of
    .0002 inches. When they got up to 300 lbs the permanent set was .0047
    inches. At 422 lbs the wire sample broke (ultimate strength).

    Mike, do you have a reference we can look at that says differently? The
    reference above was printed in 1918 and perhaps "modern" piano wire
    would be
    different.

    Dean

    Dean May             cell 812.239.3359