Quoted message said:Carl Fogel said:Hmmm... does squeezing stainless steel spokes render them immortal?
Maybe you should call it "stretching spokes" rather than squeezing,
since stretch is what is being done. This can be accomplished through
various manual and mechanical means. TREK has a method by which the
wheel is stress relieved in two operations, one side at a time.
Quoted message said:The idea is that when spokes are bent to form the elbow, potentially
fatal stresses are formed at the bend, stresses that can be relieved
by giving the tensioned spoke a good squeeze.
Quoted message said:Unfortunately, theory and data are controversial.
translation: "i have no proof & have not attempted to quantify".
Quoted message said:Not at all. As I have explained in detail (something you could find
with your skills in web searching) bending a steel wire always
involves partial spring-back, which in itself proves there are
residual stresses.
absolutely fundamentally not.
Quoted message said:If it were not the case, the wire would either
return to its original alignment or remain in the position into which
it was bent.
jobst, please please please get it into your head that spring-back is
because of the shape of the deformation graph. residual stress has
nothing to do with it. bending has nothing to do with it - you get
spring-back in linear tension samples too - and the reason, as explained
before, is that you only deform material once you're above the hookes
law part of the graph. but deforming enough for yield does not
magically allow the material to yield to zero and bypass hookes law on
the way. is there any way to explain this to you more simply? as long
as you labor under this fundamental misconception, you're always going
to keep shooting wide of the mark.
Quoted message said:
The partial spring-back results from not all depths of the cross
section having been equally deformed, the central "fiber" not having
changed length while the parts on the outside of the bend were
permanently stretched and those on the inside, compressed. In between
these extremes various amounts of plastic length change occurred.
you do get differing degrees of deformation, but they are not wholly
responsible for spring-back. see above.
Quoted message said:
I think that if you review this scenario and observe that there is
partial spring-back, that there must be residual stress.
no, no, no. that's an absolutely fundamental misconception.
Quoted message said:Quoted message said:A test or demonstration would be nice, but testing spokes is
difficult, since even unsqueezed spokes seem to last for millions of
wheel revolutions.
Get a coat hanger and start bending.
you're never going to "see" residual stress just by bending anything.
Quoted message said:Quoted message said:(Ten thousand miles is about 53 million inches, which in turn is
about two-thirds of a million spins of a 700c wheel--and 500 hours
at 20 mph. Setting up a test for a hundred plain and a hundred
squeezed spokes would be daunting.)
Forget about the miles for a moment and look at the material. As I
mentioned, placing a wire that purposely has been made wavy, in a
tensile testing machine that can tension the wire to its yield stress,
(the stress where the stress strain curve begins to flatten out) will
give a perfectly straight wire when released. This wire has no
stresses or it would spring to some other shape.
rubbish. that wire can have residual stress much as any other. the
questions is the stress orientation & whether it's significant relative
to its application. all you're describing is a material that has
yielded. there's no way you'll ever get a handle on magnitude or
relevance with this kind of hand waving.
Quoted message said:It has no reason to
take a shape other than straight because all its parts were stretched
to yield to have their collective "memories" erased. This is possible
with ductile spoke wire because it is made to undergo forming.
fundamental lack of understanding of deformation theory. the effects of
deformation are cumulative in stainless wire. the entire work history
of the material is easily visible under any metallurgical microscope.
you're not erasing anything.
Quoted message said:Quoted message said:Can anyone suggest a practical test that I can use to convince a
pack of skeptical high-school physics students, one way or the
other?
If you have a spoke in a tensioned wheel that was bent into its
in-situ shape by bending its elbow form its original obtuse angle to
an acute angle by tensioning, then it will have its outer "fibers" of
the elbow at yield stress and it will remain there because the spoke
is additionally tensioned. By stretching that spoke to nearly twice
its static tension by "stress relieving" over-tension, the outside of
the elbow must yield and when released fall back to a lower stress.
assuming you're bending the spoke elbow. if you follow the
manufacturer's instructions you wouldn't do that, and more importantly,
you'd notice that the settled angle of the spoke elbow in the hub hole,
with the "slotting" of the hole that results as the spoke seats itself,
is at about 95 degrees. which is about same as spokes are made with.
that would be a wild coincidence, wouldn't you say? we /are/ assuming
incompetent manufacturers after all...
Quoted message said:
That example is of the elbow, but it is true of the threads as well.
The elbow, however, is more obvious because outbound spokes all get
plastically deformed in the first tensioning of the wheel regardless
of their prior state.
you're hedging as to exactly which type of residual stress you think
threads have. cast the die jobst, the thread root [the critical part],
do you reckon that's tensile residual or compressive residual?
Quoted message said:Quoted message said:I think that I have the raw material, some widely available pieces
of stainless steel wire that come with three smooth u-bends from the
factory. It's so cheap that I should be ashamed to steal them in
boxes of a hundred, but the students need to learn the basics of
scrounging.
Being practical about this, I think you will see that you can do this
without a materials laboratory.
not unless you don't understand what you're looking at. to test for the
presence of residual stress, you commonly use a chemical agent that
preferentially attacks material areas with higher [crystal lattice]
energy. to measure magnitude, you commonly use an atomic distance
measuring strategy like x-ray or neutron diffraction. i don't have a
suitable neutron source in my basement. do you?
Quoted message said:Quoted message said:Any ideas about how to demonstrate over-tensioning stress-relief
using paper-clips?
I like clothes hangers better because they are large enough to clearly
see the results.
all you're seeing is bending. the fact that you've convinced yourself
that you're seeing residual stress simply illustrates just how dangerous
insufficient information or comprehension [not understanding hookes law]
can be.
Quoted message said:
http://www.sheldonbrown.com/brandt/stress-relieving.html
did you read my line-by-line critique of that article? you need to
update it jobst. but don't do it until you've been to the library to
revise your deformation theory please.
Quoted message said:
Jobst Brandt
[email hidden]
