stretching wire

[William R. Monroe]

I don't think this degree of plastic deformation stands in the way of the "strings don't stretch over the long term" idea.  First, keep in mind the amount of stretch cited here.  Even at the 300 pound tension, the plastic deformation was given as about 5 thousandths of an inch.  Maybe significant in some way, but it sure wouldn't account for any "winding up of wire on the tuning pin."  I don't know the pitch change for an increase in wire length of 5 thou for 17 ga wire, but maybe our scaling guys can tell us.  I would enjoy knowing.

More to the point, when did this plastic deformation occur, i.e., how long was the wire left tensioned?  I tried wading through this study, but didn't find the methods outlined very well.  It appears the wires were only tensioned for a short period (less than a day).  In other words, this plastic deformation of music wire occurred as the wire was initially brought to tension, and stayed there for the duration.  It does not conclude that the wire continued to deform over any length of time.

So, for our purposes, this study seems to suggest that as we chip or raise pitch, there is bound to be some plastic deformation, but that once tensioned, no more deformation occurs over the long-term.  So, the theory that strings stretch over the long-term does not seem to be supported by this study/information.

William R. Monroe


SNIP 


   

  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&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 
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