Re: spoke fatigue troll

Ben C wrote:
On 2008-04-26, jim beam <spamvortex@xxxxxxxxxxxxxxx> wrote:
Ben C wrote:
On 2008-04-26, jim beam <spamvortex@xxxxxxxxxxxxxxx> wrote:
http://www.flickr.com/photos/38636024@N00/1346747861/

your homework assignment is to annotate the above showing the location of the neutral plane, and to indicate the residual stress profile across the section.
How's this: http://i29.tinypic.com/27y4bd4.jpg
the revised one is better!


Red: tensile residual stress
Blue: compressive residual stress
Green: neutral

Based on the link Peter posted

http://www.lanl.gov/contour/beam.html
that's not a profile appropriate here [but peter cole never plays it straight] - it's a beam machined from thick section and the residual stress profile is completely different from bent wire. here's a better one:
http://www.ncnr.nist.gov/AnnualReport/FY1999/residual.pdf

bear in mind, that is not severely bent like a spoke elbow, but it's a much more relevant residual profile.

It looks like I guessed right that blue is compressive and red is
tensile.

The profiles look similar but for the thicker beam, the regions of
highest residual stress are nearer the centre of the beam. They
alternate in the same sort of way, but it's as though the effect is sort
of rippling out and becoming weaker by the time you get to the edge.

yup.



Fatigue can start in the interior,

that's /ultra/ rare.



but there are more things to get it
started on the exterior, like surface imperfections.

almost /all/ failures start at the exterior.


So you would think
tensile residual stress at the exterior would be the biggest problem--
as the coil spring has.

indeed. and on spokes too.



[...]
for a written description, try this:
http://groups.google.com/group/rec.bicycles.tech/msg/af080b93a59cca03

"cccTCttt", severely bent wire, is closer to appropriate.

That looks more like the bar-- regions of highest T (bright red) and C
(dark blue) nearer the interior.

theoretical bar, not actual. the actual on that test was somewhat skewed.



naturally, if residual stress is an initiator, you expect fatigue to start and grow from a point of high tensile residual. that's why, with a coil spring, where there is a high tensile skin residual, and where you have high skin torsion load that coincides, you do indeed observe fatigue to initiate in the high tensile region.

that's why thermal or mechanical stress relief is so important in that
application.

but with a spoke, the fatigue is observed to be independent of any high residual zone [because there's minimal applied loading there]

Are you saying spoke fatigue usually starts in the region of high
tensile loading-- i.e. the outside of the bend for outbound spokes?

either inside or outside, but definitely a region of high loading [caused by bending].


I
thought no-one really knew where it started because we haven't looked at
enough failed spokes.

who hasn't? just go to a bike shop and ask them to save some for you.




For outbound spokes the small compressive residual on the outside of the
bend will mitigate fatigue (if only a little bit).

theoretically, yes, but in practice they break pretty evenly both ways.


For inbound spokes it won't though-- they get a tensile loading on the
inside of the bend in use, where there is the small tensile residual.

Although not the highest region of tensile residual, it still might
accelerate fatigue that starts on the surface due to a surface
imperfection. I think that's Cole's point.

peter cole is just fixated on the fact that because there may be a residual stress, there will definitely be fatigue. which is not the case here because it's not observed to be initiating in that high residual zone.



That all sounds reasonably logical provided one is careful not to make
unfounded claims about how significant that tensile skin residual on the
inside of inbound spokes is compared to other factors (which to be fair
Cole doesn't).

I doubt very much that it is very significant at all, but if it were,
how about this for a wheel-building technique:

Take 18 of your 36 spokes and bend their elbows inwards to about 75
degrees (you might have to go to 65 and let them spring back).

Then bend them back out to 90 degrees (you might have to go 95 or 100
and let them spring back).

They now should have compressive residual stress on the insides of the
elbows. Perfect! Use them for the inbound spokes. The other 18 should be
good to go out of the box for the outbound spokes.

Then whatever you do DON'T stress-relieve and ruin everything.

but you can't. if you don't overload/stabilize/"stress relieve", the wheel will go out of true in a few miles as the spokes bed in to the hub holes. you can prove that to yourself next time you build. particularly rears.



Since there is now compressive residual stress on all spokes protecting
you from fatigue, I predict that they will last even longer than usual--
at least 600000 miles.

easier to just use a fatigue resistant material in the first place, or better yet, use straight pull spokes.
.

Relevant Pages

  • Re: spoke fatigue troll
    ... and to indicate the residual stress profile across ... Red: tensile residual stress ... enough failed spokes. ...
    (rec.bicycles.tech)
  • Re: vacuum de-gas thread continued
    ... Do you think a nickel a pound has any real effect on material choice for spokes or not? ... There aren't any residual stresses to relieve. ... Adding the fact that fatigue cracks start at surface defects overwhelmingly, whether those defects are inclusions, slip bands or nicks from use/fabrication, doesn't change the model. ... jobst thinks he's seeing spoke failure cause by residual stress. ...
    (rec.bicycles.tech)
  • Re: vacuum de-gas thread continued
    ... Not if you're making spokes, ... manufacturers pay the freight on vacuum degassed steel simply because of superior mechanical properties, not because they don't know how to manage their costs. ... Adding the fact that fatigue cracks start at surface defects overwhelmingly, whether those defects are inclusions, slip bands or nicks from use/fabrication, doesn't change the model. ... jobst thinks he's seeing spoke failure cause by residual stress. ...
    (rec.bicycles.tech)
  • Re: New hub from Shimano
    ... The reduced cross-section and stress concentration at the fatigue crack could cause failure. ... butbutbut, even if the failure could be put to existing fatigue cracking, that would mean getting another wheel to run the experiment on. ... Since you content your wheel with missing spokes is a functional wheel, ... these include surface finish, composition, environment, etc. and eventually residual stress. ...
    (rec.bicycles.tech)
  • Re: spoke fatigue troll
    ... of the neutral plane, and to indicate the residual stress profile across ... Blue: compressive residual stress ...
    (rec.bicycles.tech)