stretching wire

[John Delacour]

"Relaxation" does seem to be the key word to search for in relation 
to the phenomenon we are discussing.  Here's an abstract from another 
paper (1992) -- again music wire including at normal temperatures:


"
<http://www.astm.org/cgi-bin/SoftCart.exe/DIGITAL_LIBRARY/JOURNALS/TESTEVAL/PAGES/972.htm?E+mystore>

Tensile Stress Relaxation in High-Strength Spring Steel Wire

Sinha UP, Levinson DW

Abstract

Stress relaxation data were obtained in tension tests using the 
vibrating string technique (measuring the resonance frequency) in 
spring steel wire of ASTM Grade A 228 (also known as music wire). The 
steel wire of 0.56 mm diameter had a 0.29% offset tensile yield 
strength of 1689 MPa [ 93 lbs force -- JD ]. Tests were conducted at 
low temperatures in the range of 23 to 140¡C, and at initial stress 
levels of up to 75% of the 0.20% offset yield strength of the wire. 
The test duration was to 4000 h at 23¡C and to 100 h at temperatures 
to 140¡C. The test results indicate a faster rate of stress 
relaxation during the early stage of stress relaxation, and 
subsequently a slower rate of stress relaxation rate with increasing 
time, as is usually observed. An empirical equation has been 
determined that fits the experimental data very well under the given 
test conditions."

There are two ways to test this, the one above being the method I 
used in the piano leg test, where an initial load is applied to a 
wire of fixed length and the frequency of the note is plotted against 
time.  I understand that a log curve is the result.  The other way is 
to maintain a constant load and measure the extension of the wire 
over time.  The second method, apart from being more visual, strikes 
me as preferable since it relies on simple physical measurement 
rather than a frequency reading on an electronic device, unless the 
vibration of the wire can be excited perfectly reliably -- perhaps 
with a light bow rather than by plucking.

JD