At 21:20 -0500 19/4/08, kurt baxter wrote:
>If you take patented steel wire beyond its elastic
> limit, it will neck and break, perhaps right now, perhaps tomorrow
> but very soon.
>
>The point when when you go from "elastic" to "plastic" is when the wire
>permanently changes shape, never to return, right?
Yes, and with patented wire this point is very close to the breaking point
>So I would say that when a string bends around the tuning pin, and
>for sure when it dives into the becket, it changes it's shape for good.
>
>If you slightly "kink" a wire, you have taken it to it's plastic
>point... right?
Yes and no. When you make the becket on the wire the part near the
outside corner is weakened but the core and inside corner are, if
anything strengthened by compression.
>So if a "kink" type load can take the string beyond it's elastic
>point without breaking, why not "stretching" type load?
Also when the wire is initially coiled or when I straighten it for
stringing or making bass strings it takes a permanent set. Exactly
what stresses are involved I can't say, but experience suggests they
are not detrimental.
When I (or others) make bass strings I flatten the wire with a hammer
and die to hold the copper at the ends. If a string is stretched to
breaking point it will never break at the flattening but either at
the eye or at the becket or some point of friction UNLESS there is a
sharp angle at the ends of the flattened portion. When I had my
first flatteners made 25 years ago, which I still use today, we got a
chamfered end on the flattening that created too sudden a change from
the flattening to the cylindrical wire and this caused a severe
weakening. Once we had ground off and radiused the top corner to the
die, there was no problem.
These things do cause work-hardening and it's quite likely that the
flattening process actually increases the tensile strength of the
wire, but when we simply stretch the wire I think there is no
work-hardening
>Isn't there a big difference between "elastic limit" and ultimate limit?
As I've mentioned before, some people use the manufacturer's UTS as
the datum and take care not to strain the wire beyond 55% of this and
others, like me and Paulello use another figure and and take care not
to strain the wire beyond 70% of this. This allows a safety margin
needed to take into account friction points, weakening through work
(say at the eye) etc. From what I have read, the elastic limit of
patented wire is almost coterminous with the breaking strain and
there is no separate "yield point" below the breaking strain as there
is with some other materials. In other words once plastic
deformation begins the string is going to break, whereas with some
other materials the wire would stretch (irreversibly) without
breaking over a range of tensions between the yield point and the
breaking strain. We are not concerned with such materials.
The figures I use are as below, with the tension in pounds. 70% of
this figure is what I do not exceed in designing bass strings -- so
283 lbs would be the maximum tension I'd have for a covered string
with a No. 21 core.
12 0.725 174
12.5 0.750 185
13 0.775 196
13.5 0.800 207
14 0.825 218
14.5 0.850 229
15 0.875 240
15.5 0.900 251
16 0.925 264
16.5 0.950 276
17 0.975 288
17.5 1.000 300
18 1.025 314
18.5 1.050 326
19 1.075 339
19.5 1.100 339
20 1.125 371
20.5 1.150 387
21 1.175 405
21.5 1.200 421
22 1.225 439
22.5 1.250 455
23 1.300 479
23.5 1.350 504
24 1.400 532
24.5 1.450 570
25 1.500 609
25.5 1.550 660
26 1.600 726
JD
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