Whatweartrack said:Quoted message said:I stumbled across this series of pictures, diagrams, and
explanations about spoke fatigue and testing:
Quoted message said:Quoted message said:http://www.princeton.edu/~humcomp/bikes/design/desi_18.htm
http://www.princeton.edu/~humcomp/bikes/design/desi_19.htm
http://www.princeton.edu/~humcomp/bikes/design/desi_20.htm
http://www.princeton.edu/~humcomp/bikes/design/desi_21.htm
http://www.princeton.edu/~humcomp/bikes/design/desi_22.htm
Quoted message said:Quoted message said:It's taken from McMahon and Graham's "The Bicycle and the Walkman,"
Merion (1992) and indicates that testing shows that spokes crack
from the outside, beginning at surface flaws, not from the inside
where internal stresses would be located.
Quoted message said:Quoted message said:Does the testing procedure mimic real life, or does the conclusion
hold only for this kind of test?
Quoted message said:Jobst Brandt would have an authoritative answer, but my instant
observation is that the mode of testing does not appear to resemble
the type of stress cycle that a spoke is subjected to in actual
usage. As such, I question whether it has any actual applicability
to in-service failures. Personally, I don't feel that this is
useful information.
Since spokes do not bend at this place, a bending test would not be
appropriate for such an investigation in my estimation. It would have
to be a preloaded oscillating tensile test and that takes millions of
cycles as it does on a bicycle.
Quoted message said:Quoted message said:The nipple end of a bicycle spoke in a real wheel, on the other
hand, undergoes tension changes, but doesn't do much bending.
Quoted message said:And in my experience, the few failures I've seen at the nipple end
were clearly due to the spoke being stressed beyond the yield point
of the threaded section; the failure was obviously caused by tension
in excess of the spoke's load capacity. (I will note that I have
yet to see a spoke break at that point without a spokejam being
involved, also.)
I have seen many such failures and still do. In fact the one pictured
in #_18 above is classic. Such failures showed the greatest reduction
from stress relieving when Robergel spokes were the mainstay.
Quoted message said:Quoted message said:The elbow end of a bicycle spoke in a real wheel not only changes
tension, but presumably flexes a little, too.
Quoted message said:If properly stress-relieved, it flexes very little, and does so in a
manner that does not approach its yield limit. There is no reason
for it to break there absent a stress riser's presence.
Wellll, flex is a tension change except that it is a more concentrated
one than plain tension, because mainly the outer "fibers" of the bend
are affected. Stress reliving reduces the residual stress in these
fibers after the spoke has been bent into place and tensioned. I
think careful examination would show that the inside of the elbow bend
is also in tension in spite of having been bent into place.
Quoted message said:Quoted message said:I have no idea if the test was appropriate, so I'm hoping that some
engineers will look at it and try to explain what was going on. The
site and the book seem to be saying surface quality is what
matters, not the stress relief usually advocated here.
Quoted message said:From what I could see, the mode of testing that the apparatus
appeared to be using looked like it was stressing the elbow in the
opposite direction from what would actually be achieved in use. As
such, if my understanding of the information is correct, the test is
an academic exercise with no value in predicting or determining the
cause of actual in-service failures.
I think the proper way to perform such a test is to build a flange
simulation and tensile cycle the spoke under customary operating
preload. I don't see any shortcuts.
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