stretching wire -- an anecdotal analysis

[John Delacour]

At 19:24 +0100 19/4/08, Richard Brekne wrote:

>The question being thrown around here (as I understand it)  is 
>whether or not there is any plastic creep over long time when the 
>wire tension is kept well below this level of stress.

Well, let me dissociate myself from any such analysis.  I don't know 
what "plastic creep" is and I am not interested in what happens to 
wire once it has passed its limit of elasticity.

At 19:32 +0100 19/4/08, Richard Brekne wrote:

>The following link may also be helpful, as it goes directly to the
>question of whether a string can experience plastic deformation despite
>being well under yield strength limits.
>
>http://en.wikipedia.org/wiki/Creep_%28deformation%29

There is no mention of steel in the whole article and it is pretty 
clear that "creep" is not a phenomenon that occurs with patented 
steel wire or, probably, any steel wire.

If piano wire subjected to high tension _within_ its elastic limit 
gets longer over a period of days or weeks as it accommodates itself 
to the tension, then that has nothing to do with "creep" or 
"plasticity".  If you take patented steel wire beyond its elastic 
limit, it will neck and break, perhaps right now, perhaps tomorrow 
but very soon.  Whatever other people are talking about, I am not 
talking about such a case.  The case I am talking about is that, for 
example, of a length of No. 17-1/2 wire (1 mm) at a tension of 200 
lbs, a high tension but safe for this wire, and the question  is :

a) If constant tension is applied to the wire, will it, over a period 
of days get longer and then gradually settle down?

or, which comes to the same thing,

b) If 200 lbs tension is applied to the wire stretched between two 
fixed points, will the tension fall over a period of days and then 
settle down?

Experience leads me to think the answer is yes, but without an 
experiment that completely eliminates all the other causes for pitch 
drop in a piano, which have been mentioned in this thread, it is 
impossible to say it is so.  I have now devised an apparatus that 
will enable me to carry out tests and hope in the next few days to 
get the thing set up.

JD