stretching wire

[William R. Monroe]

I think everyone in this discussion is on the same page that wire does 
stretch, some plastic, some elastic.  No disagreement.  The disagreement 
seems to be over two things:

Is there is any appreciable plastic deformation after the initial tensioning 
of the string.  All the experiments cited thus far seem to suggest that is 
not the case.  None of the experiments conclusively show plastic deformation 
after initial tensioning.

Is there any elastic deformation over the long term aside from that with 
initial tensioning.  Again, all the experiments cited thus far seem to 
suggest that is not the case.  None of the experiments conclusively show 
elastic deformation after initial tensioning, the Love citation included, as 
below.

 SNIP
> I mentioned (not for the first time) the experiment quoted by A.E.H. Love 
> in his classic "A Treatise of the Mathematical Theory of Elasticity" 
> carried out by one of the French pioneers of suspension bridges, who 
> "found that wires held stretched, with a tension equal to one quarter of 
> the breaking stress, retained the length to which this tension brought 
> them throughout the whole time of his experiments (33 months)

No argument.  This deformation occured at the initial tensioning of the 
wire.

> , while similar wires stretched with a tension equal to half the breaking 
> stress, exhibited a notable gradual increase of extension."

This might be interesting if it were based upon our modern piano wire, but 
then, what is a "notable gradual increase of extension?"


SNIP

> Indeed no factors outside the string itself, I suggest, can account for a 
> piano sinking in pitch more than a full tone when left untuned for many 
> years.

I can suggest some, and I don't think that just because we haven't measured 
them we should attribute these pitch drops to long-term elastic deformation 
of piano wire.  How about pins moving in the block (rotational) how about 
pins crushing the leading edge of the block?  I'm sure there are others.  As 
noted before, the pin would only have to move about 5 thousandths of an inch 
(a green punching ;-]  ) toward the capo to give us a pitch change of about 
a semitone at 200lbs and 6" in length, for 17 ga wire, as Ron N wrote.



>
> But note well that I have not referred to "plastic deformation", and it is 
> quite clear that Vicat's wire increased in extension well below any point, 
> call it what you will, that would be considered a safe limit.  The fact 
> that a piano has sunk a tone below pitch over the years owing to this 
> gradual extension of the wire

Again, suggesting that a stretching wire is the causative effect of pitch 
drop is speculative.  Science requires we reject the null hypotheses by 
testing them not dismissing them.  We must find out if pitch drop can be 
attributed to something like moving pins, and if we prove it is not, then we 
eliminate that hypothesis, but that doesn't mean we accept the others until 
we also prove/disprove them.


> does not mean that it cannot be brought safely to pitch and quickly 
> stabilize.  I have such a piano at the moment, which I roughly pulled up 
> and rough-tuned as soon as I bought it 8 weeks ago and which has not 
> shifted since.
>

Here we are in agreement.


> So far as I understand the phenomenon, this elongation of the wire when 
> kept under tension is quite different from the permanent elongation that 
> takes place when the elastic limit has been exceeded (plastic deformation) 
> and which will lead eventually to failure.  I have had the phenomenon 
> explained to me a very clever Swiss physicist of my acquaintance, who was 
> very familiar with it.  I would have detained him longer to explain in 
> detail if I were capable of absorbing the physical and mathematical 
> details.
 SNIP
>
> JD


Agreed here as well, on two points.  The quality of plastic and elastic 
stretch differ.  And, like you, I too don't fully understand all the 
physical and mathematical details.  I only know that based upon all the 
references thus given, there has been no data presented to show long-term 
elastic deformation taking place after initial tensioning of the modern day 
piano wire.

William R. Monroe