Residual stress, fatigue and stress relief

Michael Press wrote:
In article ,
daveornee wrote:

Michael Press Wrote:
In article ,
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.
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!!!

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

Peter Cole wrote:
Ben C wrote:
On 2008-04-24, Peter Cole 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.

"jim beam" wrote:
Peter Cole wrote:
wrote:
On Apr 24, 1:07 am, Ron Ruff wrote:
On Apr 23, 4:44 pm, 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

Tom Sherman wrote:
"jim beam" wrote:
Peter Cole wrote:
wrote:
On Apr 24, 1:07 am, Ron Ruff wrote:
On Apr 23, 4:44 pm, 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.


good point. but they're both singing the same [underinformed] song.

So what do you _do_ when you are stressed and fatigued, for relief?


TBerk

On Thu, 24 Apr 2008 20:59:25 -0700, jim beam wrote:

Peter Cole wrote:
Ben C wrote:
On 2008-04-24, Peter Cole 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.



The dog returns to its vomit.

We've been over this before -- you've come up with one spoke manufacturer
-- Sapim, in a web document rife with errors. Other spoke manufacturers
disagree. Wheelsmith, for instance:

"Wheelsmith spokes are “cold headed� with great accuracy, and marked with
the distinctive “W�. Elbows are formed over a radiussed mandrel rather
than a flat surface as with other popular spokes. This results in a
larger bend radius, and preserves the round cross-section of the elbow.
Consequently, Wheelsmith spokes require a minimum of bending to conform
to a hub, thereby maximizing fatigue life."

http://www.wheelsmith.com/spokes_introduction.html




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.

Peter Cole 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.

Don't wait until then. With bright grazing incident illumination,
crazing around spoke eyelets, caused by the riveting the eyelet, is
apparent on new unused rims. They only get worse thereafter as they
propagate into the substrate and open wider on the surface.

Jobst Brandt

TBerk wrote:

So what do you _do_ when you are stressed and fatigued, for relief?

Start a flame war on Usenet?

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

wrote:
Peter Cole 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.

Don't wait until then. With bright grazing incident illumination,
crazing around spoke eyelets, caused by the riveting the eyelet, is
apparent on new unused rims. They only get worse thereafter as they
propagate into the substrate and open wider on the surface.


except that the anodizing crack orientation doesn't correlate with the
metal crack orientation!!!

but observation is a powerful tool jobst - you should be encouraged to
do this more often. a quick read on the subject of fracture mechanics
would be handy too.