Re: spoke fatigue troll

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.


Although I'm not sure if they're using negative (blue) to mean
compressive (I am) or which way they're bending their beam.

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

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

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], and indeed independent of either tensile or compressive minor residuals. therefore the largest concern is applied load, and thus its origination is in the design, not the material processing.
.

Relevant Pages

  • Re: Residual stress, fatigue and stress relief
    ... I have a spoke with a bend in it from the factory. ... stresses due to this bend. ... If, instead of bending in the same direction you bent the other way, you'd be adding to the residuals and enter the plastic mode as soon as the residual + applied exceeded the yield threshold. ... This would cause another layer of residual stress to be superimposed on the first one. ...
    (rec.bicycles.tech)
  • Re: Residual stress, fatigue and stress relief
    ... I have a spoke with a bend in it from the factory. ... stresses due to this bend. ... If, instead of bending in the same direction you bent the other way, you'd be adding to the residuals and enter the plastic mode as soon as the residual + applied exceeded the yield threshold. ... This would cause another layer of residual stress to be superimposed on the first one. ...
    (rec.bicycles.tech)