stretching wire

[Dean May]

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 

PianoRebuilders.com   812.235.5272 

Terre Haute IN  47802

 

  _____  

From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf
Of Mike Imbler
Sent: Wednesday, April 09, 2008 7:01 PM
To: pianotech at ptg.org
Subject: re: stretching wire

 

As a mechanical engineer, I concur with Ron.  John's cite of the high
tension electrical wires makes it clear that they are being tensioned beyond
yield strength, which is why the article suggests heat treating the wire
(which increases yield strength).  Yield strength is the point where the
wire would plastically deform (stretch) which is less than ultimate strength
(breaking stress).  Therefore, you -could- stretch piano wire, but you would
be pulling it above its yield strength.  This would be more likely with more
ductile wire which has a yield strength appreciably lower than ultimate.
With a more modern piano wire, yield strength is closer to ultimate.

              Mike 

-------------- next part --------------
An HTML attachment was scrubbed...
URL: https://www.moypiano.com/ptg/pianotech.php/attachments/20080410/de753b34/attachment.html