Residual stress, fatigue and stress relief

[Michael Press]

In article <[email protected]>,
daveornee <[email protected]> wrote:

> 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.

Thereby putting the wheel out of true.
Spoke wind up is never present when
attention is paid while turning the
nipples. Turn until the nipple turns
with respect to the spoke, then back
the spoke wrench until the spoke is
not wound up.


> The process also
> temporarily overloads spokes in the direction of the working load....

No it does not. All strain is elastic when the
spoke wrench turns the nipple.

> 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...

No it won't. Nipples unwind when the spoke tension is insufficient
to the job of carrying the cyclic load at the contact patch.

> sometimes to the
> point where nipples will back-off and the wheel will have spokes that
> are totally slack.

Oil the threads and spoke bed. Use a sufficient number
of spokes to carry the load.

--
Michael Press

 

[daveornee]

In article <[email protected]>,
daveornee <[email protected]> wrote:

> 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.

Thereby putting the wheel out of true.
Spoke wind up is never present when
attention is paid while turning the
nipples. Turn until the nipple turns
with respect to the spoke, then back
the spoke wrench until the spoke is
not wound up.


> The process also
> temporarily overloads spokes in the direction of the working load....

No it does not. All strain is elastic when the
spoke wrench turns the nipple.

> 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...

No it won't. Nipples unwind when the spoke tension is insufficient
to the job of carrying the cyclic load at the contact patch.

> sometimes to the
> point where nipples will back-off and the wheel will have spokes that
> are totally slack.

Oil the threads and spoke bed. Use a sufficient number
of spokes to carry the load.

--
Michael Press

I build my way and it always works. I properly lubricate the threads and I make sure there isn't windup as I tighten nipples.
If the wheel goes out of true while stabilizing it isn't likely to stay true when riding.
I use the proper numbers of spokes for the intended loads in my builds.
I don't uderstand what you mean:
"No it does not. All strain is elastic when the spoke wrench turns the nipple."
Pick apart as you want!

 

[jim beam]

[email protected] wrote:
> On Apr 23, 4:20 pm, Peter Cole <[email protected]> wrote:
>> [email protected] wrote:
>>> This is because residual compression on one side of the bend is
>>> residual tension on the other, and trying to produce just the right
>>> amount of residual stress in a spoke by hand is like aligning
>>> microscope lenses with a framing hammer. It works great as long as
>>> you never look into the eye piece.
>> It doesn't matter for the overload method of stress relief. That's what
>> makes it such a useful technique.
>
> So the magnitude doesn't matter?

of course they do. peter cole is just clutching at straws at this point.


> Proof loading of things like gun
> barrels is done to a fairly precise stress level. To think that
> without doing any calculations on spoke yielding and cracking or using
> any instrumentation in application other than your bare hands will
> achieve this level of precision is absurd. I'm willing to believe
> that residual stresses may improve fatigue life under the right proof
> loading conditions, but I will not accept that the average wheel
> builder (myself included) has taken the time to figure out exactly
> what those conditions are and ensured that they're being followed.

 

[jim beam]

daveornee wrote:
> 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.
>
>


indeed. and it's easy to test this observation at home - it's not like
this is inaccessible rocket surgery.

 

[jim beam]

Peter Cole wrote:
> 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.

er, actually, the residual stress at the root of cold rolled threads is
compressive, not tensile. you *don't* want to "relieve" that.

and your cite is for the phenomenon of residual stress and mechanical
stress relief. nobody contests that it exists. what's being contested,
and the point you /still/ don't seem to be able to grasp, is that
OBSERVATION OF THE FACTS contradicts the supposition that it plays any
material role in spoke fatigue. supposition, not matter how hotly
defended, is simply not fact, and no amount of repetition by either you
or jobst can make it so.

 

[jim beam]

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.

jobstian "stress relief" theory is presumptive nonsense that has no
correlation with observed fact. it's as well grounded as his assumption
that fatigue can be eliminated from spokes made of stainless steel, a
material with no fatigue endurance limit.

absent sufficient knowledge and absent proper application of scientific
method, you're just perpetuating jobstian mistakes - and looking foolish
to boot.

 

[Ron Ruff]

On Apr 23, 4:44 pm, [email protected] wrote:
> So the magnitude doesn't matter?

I suppose it would be nice to be more precise about this, but I'm not
sure it matters a whole lot. I'm presently thinking that more is
better for spokes... bending spokes at the hub, cross, and nipple, and
then stressing them as much as I can after full (and even excessive)
final build tension is acheived. Pressing on the spokes with the wheel
laying on a soft surface seems to work best. Pressing on the rim will
also work except for DS and carbon rims.

 

[[email protected]]

On Apr 24, 1:07 am, Ron Ruff <[email protected]> wrote:
> On Apr 23, 4:44 pm, [email protected] wrote:
>
> > So the magnitude doesn't matter?
>
> I suppose it would be nice to be more precise about this, but I'm not
> sure it matters a whole lot. I'm presently thinking that more is
> better for spokes... bending spokes at the hub, cross, and nipple, and
> then stressing them as much as I can after full (and even excessive)
> final build tension is acheived. Pressing on the spokes with the wheel
> laying on a soft surface seems to work best. Pressing on the rim will
> also work except for DS and carbon rims.

It only matters if your "more is better" is enough to initiate cracks,
but not enough to immediately fracture the whole spoke.

 

[Peter Cole]

Ben C wrote:
> On 2008-04-23, Peter Cole <[email protected]> wrote:

>> 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.

If there are residual stresses, there must be both tension and
compression residuals. Since the working load is tension, tension
residuals are bad for fatigue, compression residuals are harmless or
beneficial. By overloading in the working direction, tension residuals
-- wherever they occur -- are reduced.

Additionally, notches act as stress concentrators, so will multiply the
working stress. By overloading, the immediate vicinity yields, creating
a beneficial compression residual.

All of the above statements are true. Stress relief can only improve
fatigue life, whether the root cause of premature failure is residual
stress or notches (defects).

The only other possible cause for early spoke fatigue is the presence of
a bending moment in the working load. If the spoke is properly formed
and supported this won't exist. If it did, the peak stress would be at
the skin, and an overload would yield those spots close to yield (just
as it would near a notch). Again, identically, it would improve fatigue
life.

All 3 of the above scenarios are not speculative.

A speculative scenario that has been proposed is that the overload "beds
in" the spoke heads to the (aluminum, presumably) hub flange, by
deforming the material. Consider that the overload is only (nominally)
50%. We are to believe that the spoke head will sink further into the
beveled hole in the flange? If it did, what benefit would that yield?
Perhaps it's the "bedding" of the spoke into the hole bore? The spoke
shaft into the hub flange? The theory (presumably) is that the spoke
elbow is not well supported initially (after tensioning) and that the
50% overload will deform the hub enough to change the elbow support
enough to remove a significant amount of bending moment.

Problems with the above speculation:

It assumes that the 50% overload somehow achieves a critical amount of
hole deformation that the initial tension didn't.

It assumes that the hole wall deforms, but the spoke doesn't plastically
yield (bend further).

Assuming the flange holes are parallel to the axle (check it if you
don't believe -- I did), and that spoke elbows are initially formed (at
the factory) to 90-95 degrees, that angle can't be optimal for both in
and out bound spokes (bracing angle +-5-8 degrees), the speculation
assumes that the out bound hole is wallowed (by shear force*) to match
the spoke angle, while the in bound hole (subject to the same shear
force) is not. If they both deform to the same degree, then the angular
mismatch remains, and a bending moment remains. *must be shear, else
spoke would bend.

If spoke tension lowers after stress relief (commonly observed) and the
spokes are not unwinding (or unscrewing), then something must have
yielded. The spoke can not have stretched (insufficient force), so the
candidates are: spoke interfaces (hub, nipple/rim) or spoke bending
(elbow, nipple). The "hub bedding" speculation assumes that the force is
high enough to deform (additionally) the hole edge (and change the
"bedding angle") without changing the spoke elbow angle (bending). How
can the bedding angle change while the spoke elbow angle doesn't? It
can't. If the bedding angle changes while the spoke is being tensioned
and/or stress relieved, the spoke elbow angle must change, too (it is in
contact). It doesn't really matter if the change is plastic (it is,
since elbows take a set), since even an elastic change would leave the
skin close to yield. Stress relieving adds a tension component to the
bending stress, yielding the areas brought close to yield from the
bending stress.

It doesn't matter how the overload is applied, only that it be in the
same direction as the working load. It will reduce operating stresses
whether from residual manufacturing stress, stress raisers or skin
stress from (static) bending moments.

 

[Peter Cole]

[email protected] wrote:
> On Apr 24, 1:07 am, Ron Ruff <[email protected]> wrote:
>> On Apr 23, 4:44 pm, [email protected] wrote:
>>
>>> So the magnitude doesn't matter?
>> I suppose it would be nice to be more precise about this, but I'm not
>> sure it matters a whole lot. I'm presently thinking that more is
>> better for spokes... bending spokes at the hub, cross, and nipple, and
>> then stressing them as much as I can after full (and even excessive)
>> final build tension is acheived. Pressing on the spokes with the wheel
>> laying on a soft surface seems to work best. Pressing on the rim will
>> also work except for DS and carbon rims.
>
> It only matters if your "more is better" is enough to initiate cracks,
> but not enough to immediately fracture the whole spoke.

"Initiate cracks" with a single load cycle to ~50% yield? Please, let's
be serious.

 

[Ben C]

On 2008-04-24, Peter Cole <[email protected]> wrote:
> Ben C wrote:
>> On 2008-04-23, Peter Cole <[email protected]> wrote:
>
>>> 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.
>
> If there are residual stresses, there must be both tension and
> compression residuals. Since the working load is tension, tension
> residuals are bad for fatigue, compression residuals are harmless or
> beneficial. By overloading in the working direction, tension residuals
> -- wherever they occur -- are reduced.
>
> Additionally, notches act as stress concentrators, so will multiply the
> working stress. By overloading, the immediate vicinity yields, creating
> a beneficial compression residual.
>
> All of the above statements are true. Stress relief can only improve
> fatigue life, whether the root cause of premature failure is residual
> stress or notches (defects).

OK, yes I think I agree with that-- that it is unlikely to make the
residual stress situation worse than it was before in any way.

> The only other possible cause for early spoke fatigue is the presence of
> a bending moment in the working load.

That's my prime suspect.

> If the spoke is properly formed
> and supported this won't exist. If it did, the peak stress would be at
> the skin, and an overload would yield those spots close to yield (just
> as it would near a notch). Again, identically, it would improve fatigue
> life.

Yes, by bending the spoke (or crushing the hub) a bit thereby reducing
the moment.

> All 3 of the above scenarios are not speculative.
>
> A speculative scenario that has been proposed is that the overload "beds
> in" the spoke heads to the (aluminum, presumably) hub flange, by
> deforming the material.

That is possible. I agree that it is speculative. We know that hub holes
deform, but not that that happens during wheel stabilization rather than
in the earlier stages of tensioning.

> Consider that the overload is only (nominally)
> 50%. We are to believe that the spoke head will sink further into the
> beveled hole in the flange? If it did, what benefit would that yield?

Reducing moment.

> Perhaps it's the "bedding" of the spoke into the hole bore? The spoke
> shaft into the hub flange? The theory (presumably) is that the spoke
> elbow is not well supported initially (after tensioning) and that the
> 50% overload will deform the hub enough to change the elbow support
> enough to remove a significant amount of bending moment.

You got it!

> Problems with the above speculation:
>
> It assumes that the 50% overload somehow achieves a critical amount of
> hole deformation that the initial tension didn't.

Agreed.

> It assumes that the hole wall deforms, but the spoke doesn't plastically
> yield (bend further).

Well, I think the spoke probably _does_ bend further. Stabilization is
justa another way of improving spoke line. Most likely scenario in my
opinion is that it does that by bending the elbow, but it is also
possible (though more speculative) that the hub may yield a bit too at
this point.

> Assuming the flange holes are parallel to the axle (check it if you
> don't believe -- I did), and that spoke elbows are initially formed (at
> the factory) to 90-95 degrees, that angle can't be optimal for both in
> and out bound spokes (bracing angle +-5-8 degrees), the speculation
> assumes that the out bound hole is wallowed (by shear force*) to match
> the spoke angle, while the in bound hole (subject to the same shear
> force) is not. If they both deform to the same degree, then the angular
> mismatch remains, and a bending moment remains. *must be shear, else
> spoke would bend.
>
> If spoke tension lowers after stress relief (commonly observed) and the
> spokes are not unwinding (or unscrewing), then something must have
> yielded. The spoke can not have stretched (insufficient force), so the
> candidates are: spoke interfaces (hub, nipple/rim) or spoke bending
> (elbow, nipple). The "hub bedding" speculation assumes that the force is
> high enough to deform (additionally) the hole edge (and change the
> "bedding angle") without changing the spoke elbow angle (bending). How
> can the bedding angle change while the spoke elbow angle doesn't? It
> can't.

It depends how small the moment is. The smaller the moment, the more
deformation of the hub can (possibly) occur.

> If the bedding angle changes while the spoke is being tensioned
> and/or stress relieved, the spoke elbow angle must change, too (it is in
> contact). It doesn't really matter if the change is plastic (it is,
> since elbows take a set), since even an elastic change would leave the
> skin close to yield. Stress relieving adds a tension component to the
> bending stress, yielding the areas brought close to yield from the
> bending stress.
>
> It doesn't matter how the overload is applied, only that it be in the
> same direction as the working load. It will reduce operating stresses
> whether from residual manufacturing stress, stress raisers or skin
> stress from (static) bending moments.

I agree with all that, I just think the last of those three is likely to
be the significant one.

Another question I've asked a few times but don't remember anyone
answering satisfactorily (my apologies if they have) is what happens to
these residual manufacturing stresses after you correct the spoke line?

I have a spoke with a bend in it from the factory. There are residual
stresses due to this bend. So far so good. Then I bend it some more to
change the angle. What has now happened to the original residual
stresses?

 

[Peter Cole]

Ben C wrote:
> On 2008-04-24, Peter Cole <[email protected]> wrote:

>> It doesn't matter how the overload is applied, only that it be in the
>> same direction as the working load. It will reduce operating stresses
>> whether from residual manufacturing stress, stress raisers or skin
>> stress from (static) bending moments.
>
> I agree with all that, I just think the last of those three is likely to
> be the significant one.

Well, you're entitled to your opinions, but stress relief will be
beneficial in all cases. The overload technique will reduce all tension
stress in the tensioned wheel that are close to yield.

Since the spoke line can't be initially right for both out & in bound
spokes, and, to the extent that hole deformation occurs, it helps one &
harms the other, the spoke line must be corrected. That will introduce
additional residual stresses, which need be relieved. If there is any
residual mismatch between spoke elbow angle and spoke bed angle, that
will introduce static stresses. These are reduced by stress relieving,
whether there is additional hole deformation or not.

 

[Michael Press]

In article <[email protected]>,
daveornee <[email protected]> wrote:

> Michael Press Wrote:
> > In article <[email protected]>,
> > daveornee <[email protected]> wrote:
> >
> > > 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.
> >
> > Thereby putting the wheel out of true.
> > Spoke wind up is never present when
> > attention is paid while turning the
> > nipples. Turn until the nipple turns
> > with respect to the spoke, then back
> > the spoke wrench until the spoke is
> > not wound up.
> >
> >
> > > The process also
> > > temporarily overloads spokes in the direction of the working
> > load....
> >
> > No it does not. All strain is elastic when the
> > spoke wrench turns the nipple.
> >
> > > 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...
> >
> > No it won't. Nipples unwind when the spoke tension is insufficient
> > to the job of carrying the cyclic load at the contact patch.
> >
> > > sometimes to the
> > > point where nipples will back-off and the wheel will have spokes
> > that
> > > are totally slack.
> >
> > Oil the threads and spoke bed. Use a sufficient number
> > of spokes to carry the load.
> >
> I build my way and it always works. I properly lubricate the threads
> and I make sure there isn't windup as I tighten nipples.
> If the wheel goes out of true while stabilizing it isn't likely to stay
> true when riding.
> I use the proper numbers of spokes for the intended loads in my
> builds.
> I don't uderstand what you mean:
> "No it does not. All strain is elastic when the spoke wrench turns the
> nipple."

I was confused when you said "the process",
and take back what I said there.


[...]

--
Michael Press

 

[Peter Cole]

Ben C wrote:

> Another question I've asked a few times but don't remember anyone
> answering satisfactorily (my apologies if they have) is what happens to
> these residual manufacturing stresses after you correct the spoke line?
>
> I have a spoke with a bend in it from the factory. There are residual
> stresses due to this bend. So far so good. Then I bend it some more to
> change the angle. What has now happened to the original residual
> stresses?

Depends how much you bend it and in which direction.

Check out the graph:
http://www.lanl.gov/contour/beam.html

Typical residual stress profile for a bent beam (TCTC).

If you consider that spoke wire has some amount of spring back, to get
the original ~95 degree angle, the spoke must have been bent to around
85 and sprang back 10. If you bent the spoke to that original 85, you'd
be just to the point of creating additional plastic deformation. If you
bent it the other way instead, opposite the original bend, you would not
have to go as far to get into the plastic region because the residual
stresses have already got you part way there.

If you bend beyond the 85 (to 80 say), you will deform (plastic)
material from the skin down to beyond the original elastic/plastic
deformation transition boundary. After relaxing, the residual profile
would be similar, but the stress reversal layer would be deeper in the
material.

If, instead of bending in the same direction you bent the other way (95
to 100 say), you'd be adding to the residuals and enter the plastic mode
as soon as the residual + applied exceeded the yield threshold. At
that point, beginning at the skin, material would plastically deform in
an opposite fashion to the original (manufacturing) deformation. Skin
that had been compressed would be stretched and vice versa. This would
cause (after relaxation) another layer of residual stress to be
superimposed on the first one. You will have restored some of the
material (near the skin) to its original unbent state (more or less),
but not the deeper layers.

If, instead of bending either way, you just load the spoke with pure
tension, you will superimpose that stress on the residual stress curve,
shifting the whole thing upward. If the residual peaks (tension) go
beyond yield with this added stress, they will plastically deform. When
you relax, you will have "knocked off the tops" of the residual stress
profile.

 

[jim beam]

Peter Cole wrote:
> Ben C wrote:
>
>> Another question I've asked a few times but don't remember anyone
>> answering satisfactorily (my apologies if they have) is what happens to
>> these residual manufacturing stresses after you correct the spoke line?
>>
>> I have a spoke with a bend in it from the factory. There are residual
>> stresses due to this bend. So far so good. Then I bend it some more to
>> change the angle. What has now happened to the original residual
>> stresses?
>
> Depends how much you bend it and in which direction.
>
> Check out the graph:
> http://www.lanl.gov/contour/beam.html
>
> Typical residual stress profile for a bent beam (TCTC).
>
> If you consider that spoke wire has some amount of spring back, to get
> the original ~95 degree angle, the spoke must have been bent to around
> 85 and sprang back 10. If you bent the spoke to that original 85, you'd
> be just to the point of creating additional plastic deformation. If you
> bent it the other way instead, opposite the original bend, you would not
> have to go as far to get into the plastic region because the residual
> stresses have already got you part way there.
>
> If you bend beyond the 85 (to 80 say), you will deform (plastic)
> material from the skin down to beyond the original elastic/plastic
> deformation transition boundary. After relaxing, the residual profile
> would be similar, but the stress reversal layer would be deeper in the
> material.
>
> If, instead of bending in the same direction you bent the other way (95
> to 100 say), you'd be adding to the residuals and enter the plastic mode
> as soon as the residual + applied exceeded the yield threshold. At that
> point, beginning at the skin, material would plastically deform in an
> opposite fashion to the original (manufacturing) deformation. Skin that
> had been compressed would be stretched and vice versa. This would cause
> (after relaxation) another layer of residual stress to be superimposed
> on the first one. You will have restored some of the material (near the
> skin) to its original unbent state (more or less), but not the deeper
> layers.
>
> If, instead of bending either way, you just load the spoke with pure
> tension, you will superimpose that stress on the residual stress curve,
> shifting the whole thing upward. If the residual peaks (tension) go
> beyond yield with this added stress, they will plastically deform. When
> you relax, you will have "knocked off the tops" of the residual stress
> profile.

and /where/ exactly does fatigue initiate???

 

[jim beam]

Peter Cole wrote:
> [email protected] wrote:
>> On Apr 24, 1:07 am, Ron Ruff <[email protected]> wrote:
>>> On Apr 23, 4:44 pm, [email protected] wrote:
>>>
>>>> So the magnitude doesn't matter?
>>> I suppose it would be nice to be more precise about this, but I'm not
>>> sure it matters a whole lot. I'm presently thinking that more is
>>> better for spokes... bending spokes at the hub, cross, and nipple, and
>>> then stressing them as much as I can after full (and even excessive)
>>> final build tension is acheived. Pressing on the spokes with the wheel
>>> laying on a soft surface seems to work best. Pressing on the rim will
>>> also work except for DS and carbon rims.
>>
>> It only matters if your "more is better" is enough to initiate cracks,
>> but not enough to immediately fracture the whole spoke.
>
> "Initiate cracks" with a single load cycle to ~50% yield?

absolutely.


> Please, let's
> be serious.

you be serious michael - look up miner's cumulative damage rule. and
"yield" is not "yield" - fatigue simply would not happen if there was no
yielding before "yield".

[do you want to discuss why most metals don't obey hooke's law now?]

 

[jim beam]

Michael Press wrote:
> In article <[email protected]>,
> daveornee <[email protected]> wrote:
>
>> Michael Press Wrote:
>>> In article <[email protected]>,
>>> daveornee <[email protected]> wrote:
>>>
>>>> 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.
>>> Thereby putting the wheel out of true.
>>> Spoke wind up is never present when
>>> attention is paid while turning the
>>> nipples. Turn until the nipple turns
>>> with respect to the spoke, then back
>>> the spoke wrench until the spoke is
>>> not wound up.
>>>
>>>
>>>> The process also
>>>> temporarily overloads spokes in the direction of the working
>>> load....
>>>
>>> No it does not. All strain is elastic when the
>>> spoke wrench turns the nipple.
>>>
>>>> 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...
>>> No it won't. Nipples unwind when the spoke tension is insufficient
>>> to the job of carrying the cyclic load at the contact patch.
>>>
>>>> sometimes to the
>>>> point where nipples will back-off and the wheel will have spokes
>>> that
>>>> are totally slack.
>>> Oil the threads and spoke bed. Use a sufficient number
>>> of spokes to carry the load.
>>>
>> I build my way and it always works. I properly lubricate the threads
>> and I make sure there isn't windup as I tighten nipples.
>> If the wheel goes out of true while stabilizing it isn't likely to stay
>> true when riding.
>> I use the proper numbers of spokes for the intended loads in my
>> builds.
>> I don't uderstand what you mean:
>> "No it does not. All strain is elastic when the spoke wrench turns the
>> nipple."
>
> I was confused when you said "the process",
> and take back what I said there.
>
>
> [...]
>


"confused"??? that's gotta be potm!!!

 

[jim beam]

Peter Cole wrote:
> Ben C wrote:
>> On 2008-04-23, Peter Cole <[email protected]> wrote:
>
>>> 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.
>
> If there are residual stresses, there must be both tension and
> compression residuals. Since the working load is tension, tension
> residuals are bad for fatigue, compression residuals are harmless or
> beneficial. By overloading in the working direction, tension residuals
> -- wherever they occur -- are reduced.

what's the point of bothering if it's not causing fatigue??? you can't
just presume a problem then prescribe a cure. well you can, but you're
wasting everybody's time, including your own. and you look stupid to
people that know better.


>
> Additionally, notches act as stress concentrators, so will multiply the
> working stress.

depending on orientation and location.



> By overloading, the immediate vicinity yields, creating
> a beneficial compression residual.

it depends where. and more to the point, it depends whether it's worth
it if it's not causing fatigue, as is the case with spokes.


>
> All of the above statements are true. Stress relief can only improve
> fatigue life, whether the root cause of premature failure is residual
> stress or notches (defects).

actually,

1. overload can introduce more residual stress than it relieves.
2. miner's cumulative damage rule says you're simply reducing fatigue life.
3. you can actually *activate* dislocation movement and thus initiate
fatigue.


>
> The only other possible cause for early spoke fatigue is the presence of
> a bending moment in the working load.

it's not the "other" cause, it's /the/ cause. a simple examination of
any broken spoke will tell you that.


> If the spoke is properly formed
> and supported this won't exist.

rubbish. any elbowed spoke has a bending moment - by definition.


> If it did, the peak stress would be at
> the skin, and an overload would yield those spots close to yield (just
> as it would near a notch). Again, identically, it would improve fatigue
> life.

you're confused.


>
> All 3 of the above scenarios are not speculative.

but only one is observed to be relevant!



>
> A speculative scenario that has been proposed is that the overload "beds
> in" the spoke heads to the (aluminum, presumably) hub flange, by
> deforming the material. Consider that the overload is only (nominally)
> 50%. We are to believe that the spoke head will sink further into the
> beveled hole in the flange?

of course. do you understand that metals yield when over-loaded?


<snip remaining boring confused garbage>

 

[jim beam]

Peter Cole wrote:
> Ben C wrote:
>> On 2008-04-24, Peter Cole <[email protected]> wrote:
>
>>> It doesn't matter how the overload is applied, only that it be in the
>>> same direction as the working load. It will reduce operating stresses
>>> whether from residual manufacturing stress, stress raisers or skin
>>> stress from (static) bending moments.
>>
>> I agree with all that, I just think the last of those three is likely to
>> be the significant one.
>
> Well, you're entitled to your opinions, but stress relief will be
> beneficial in all cases.

no. three reasons cited above.


> The overload technique will reduce all tension
> stress in the tensioned wheel that are close to yield.
>
> Since the spoke line can't be initially right for both out & in bound
> spokes,

untrue. canted flanges. ever heard of them?


> and, to the extent that hole deformation occurs, it helps one &
> harms the other, the spoke line must be corrected. That will introduce
> additional residual stresses, which need be relieved.

manufacturers actually state that spokes /shouldn't/ be bent.


> If there is any
> residual mismatch between spoke elbow angle and spoke bed angle, that
> will introduce static stresses. These are reduced by stress relieving,
> whether there is additional hole deformation or not.

all of which presumptive nonsense ignorantly ignores the fundamental
fact that fatigue cracking is not observed to be initiating in regions
of high residual stress!!! no matter how you try michael, you can't fit
a 3" turd into a 1" pipe, just like you can't fudge facts to fit a
misconceived mistake.

 

[Tom Sherman]

"jim beam" wrote:
> Peter Cole wrote:
>> [email protected] wrote:
>>> On Apr 24, 1:07 am, Ron Ruff <[email protected]> wrote:
>>>> On Apr 23, 4:44 pm, [email protected] wrote:
>>>>
>>>>> So the magnitude doesn't matter?
>>>> I suppose it would be nice to be more precise about this, but I'm not
>>>> sure it matters a whole lot. I'm presently thinking that more is
>>>> better for spokes... bending spokes at the hub, cross, and nipple, and
>>>> then stressing them as much as I can after full (and even excessive)
>>>> final build tension is acheived. Pressing on the spokes with the wheel
>>>> laying on a soft surface seems to work best. Pressing on the rim will
>>>> also work except for DS and carbon rims.
>>>
>>> It only matters if your "more is better" is enough to initiate cracks,
>>> but not enough to immediately fracture the whole spoke.
>>
>> "Initiate cracks" with a single load cycle to ~50% yield?
>
> absolutely.
>
>
>> Please, let's be serious.
>
> you be serious michael[...]

How does "Peter Cole" equate to "michael"? The world wonders.

--
Tom Sherman - Holstein-Friesland Bovinia
The weather is here, wish you were beautiful