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Residual stress, fatigue and stress relief



 
 
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  #11  
Old April 23rd 08, 09:11 PM posted to rec.bicycles.tech
Leo Lichtman
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Default Residual stress, fatigue and stress relief


wrote:
This is because residual compression on one side of the bend is
residual tension on the other, (clip)

^^^^^^^^^^^^^^^^
No. You are evidently applying the equations for bending stress, with
symmetry about the neutral axis. If there are tensile stresses present, the
bending stresses add on one side and subtract on the other. Then, if the
higher value (either tensile or compressive) passes the yield point, the
symmetry is gone, and the residual stress could have an effect on fatigue
afterward.


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  #12  
Old April 23rd 08, 09:16 PM posted to rec.bicycles.tech
Peter Cole[_2_]
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Default Residual stress, fatigue and stress relief

Ben C wrote:

The controversy here is not that brief overload relieves stress or that
stress relief improves fatigue life.


Not true.

It's the claim that this is known
to be _the significant beneficial effect_ of spoke-squeezing, the Mavic
method, and other "stabilization" practices that people do when
wheel-building.


Not true. The specific claim (originally by Jobst) is that spoke
squeezing causes stress relief by the exact mechanism described in the
sources I cited. "Stabilization" is your word -- and a meaningless one,
too. Stress relief is a specific term. That there are residual stresses
in spokes is not a matter of faith. Overloading in the direction of the
working load will either diminish undesirable residual stresses or
create desirable residual stresses or both. That is the whole point. It
needs no other qualifications.
  #15  
Old April 23rd 08, 09:30 PM posted to rec.bicycles.tech
Peter Cole[_2_]
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Default Residual stress, fatigue and stress relief

Ben C wrote:

I don't remember Jobst mentioning anything about this mechanism of
notches resulting in compressive residual stress but never mind.


Never mind, yourself. If threads aren't notches, I don't know what are.
Jobst claimed that his technique of stress relief would improve failure
rates at the threads, too. The published material I cited supports this
claim.
  #17  
Old April 23rd 08, 09:49 PM posted to rec.bicycles.tech
Peter Cole[_2_]
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Default Residual stress, fatigue and stress relief

Ben C wrote:

I think jim's point is that no-one has shown that spoke fatigue starts
on the inside of the bend significantly (or at all) more often than it
starts on the outside.


"jim beam" doesn't correct his spoke lines. He will have a moment under
normal load. That will likely be larger than any residual stress
contribution.

The evidence we would expect to see for residual stress being a factor
just isn't there.

Having said that many people (who aren't jim beam) don't scrutinize the
broken spoke carefully through a magnifying glass, but just chuck it in
the trash, so we wouldn't know.


People like Jobst & I are at a disadvantage from a "forensics" angle --
we don't break spokes, so have no samples to analyze. "jim beam" seems
to have plenty. Follow his faith-based analysis if you want.
  #18  
Old April 23rd 08, 10:04 PM posted to rec.bicycles.tech
Ben C
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Default Residual stress, fatigue and stress relief

On 2008-04-23, Peter Cole wrote:
Ben C wrote:

The controversy here is not that brief overload relieves stress or that
stress relief improves fatigue life.


Not true.

It's the claim that this is known
to be _the significant beneficial effect_ of spoke-squeezing, the Mavic
method, and other "stabilization" practices that people do when
wheel-building.


Not true. The specific claim (originally by Jobst) is that spoke
squeezing causes stress relief by the exact mechanism described in the
sources I cited. "Stabilization" is your word -- and a meaningless one,
too.


The idea is to choose a term that does not rule out that spoke squeezing
(etc.) may have a beneficial effect but that isn't because it relieves
residual stress.

Stress relief is a specific term. That there are residual stresses
in spokes is not a matter of faith.


No, but that they make any practical difference to how quickly the spoke
breaks or not is.
  #19  
Old April 23rd 08, 10:35 PM posted to rec.bicycles.tech
Ben C
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Posts: 3,084
Default Residual stress, fatigue and stress relief

On 2008-04-23, Peter Cole wrote:
Ben C wrote:

I think jim's point is that no-one has shown that spoke fatigue starts
on the inside of the bend significantly (or at all) more often than it
starts on the outside.


"jim beam" doesn't correct his spoke lines. He will have a moment under
normal load. That will likely be larger than any residual stress
contribution.


The presence of moment under normal load is my preferred likely
candidate for what causes most spoke failures.

And in fact part of what Jobst calls "residual stress" is I believe
just a convoluted description of moment under load.

He says that when you put the bend in the elbow of an outbound spoke the
bend can't spring back because the flange is in the way but is held
there by spoke tension. He calls that "residual stress", which is
confusing, but that's what he calls it.

For that to be possible there must be quite a bit of moment, since spoke
tension is not high enough to hold a bend in an elbow that is close to
flush with the flange. Overloading reduces that tension by bending the
elbow and/or deforming the hub and reducing the moment. Jobst says
instead overloading deforms the fibres or something and relieves the
"residual stress". But it is actually basically the same thing (although
I think Jobst denies the hub can possibly deform at this point).

That is my conclusion from all the discussions I have read about this on
RBT. Residual stress deserves mention as a possible mechanism that may
also be involved, but no more than that.

Beam is right: straight-pull spokes are a better design. On the other
hand with aluminium hubs and quality steel spokes breakages are rare
these days so it's not really as big a problem as a casual reader of RBT
might take it for.

The evidence we would expect to see for residual stress being a factor
just isn't there.

Having said that many people (who aren't jim beam) don't scrutinize the
broken spoke carefully through a magnifying glass, but just chuck it in
the trash, so we wouldn't know.


People like Jobst & I are at a disadvantage from a "forensics" angle --
we don't break spokes, so have no samples to analyze.


I thought Jobst did used to break them before he started
stress-relieving.

"jim beam" seems to have plenty. Follow his faith-based analysis if
you want.


I know you don't really want to be a troll. Fight the urge.
  #20  
Old April 23rd 08, 11:09 PM posted to rec.bicycles.tech
daveornee[_184_]
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Posts: 1
Default Residual stress, fatigue and stress relief


Peter Cole Wrote:
Ben C wrote:

The controversy here is not that brief overload relieves stress or

that
stress relief improves fatigue life.


Not true.

It's the claim that this is known
to be _the significant beneficial effect_ of spoke-squeezing, the

Mavic
method, and other "stabilization" practices that people do when
wheel-building.


Not true. The specific claim (originally by Jobst) is that spoke
squeezing causes stress relief by the exact mechanism described in the
sources I cited. "Stabilization" is your word -- and a meaningless
one,
too. Stress relief is a specific term. That there are residual
stresses
in spokes is not a matter of faith. Overloading in the direction of
the
working load will either diminish undesirable residual stresses or
create desirable residual stresses or both. That is the whole point.
It
needs no other qualifications.

"Stabilizing" is a term used Barnett Bicycle Institute in thier wheel
building classes. It is not meaningless. Stabilizing makes sure the
spokes are embeded and residual windup removed. The process also
temporarily overloads spokes in the direction of the working load....
likely better than the spoke squeezing method. I can see and
immediately measure the results of the process. I know that if a wheel
isn't stabilized it will detension as it is ridden... sometimes to the
point where nipples will back-off and the wheel will have spokes that
are totally slack.


--
daveornee

 




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