Residual stress, fatigue and stress relief

Peter Cole said:

"Metal Fatigue in Engineering", Ralph I. Stephens, Ali Fatemi, Robert R.
Stephens, Henry O. Fuchs, Ali Faterni
http://www.amazon.com/gp/reader/0471510599/ref=sib_dp_pop_toc?ie=UTF8&p=S008#reader-link

Page 247-8 describe creation of beneficial residual stress at a notch by
overloading in tension, whereby the stress concentrating effect of the
notch brings the material above yield in the immediate notch vicinity,
followed by a residual compressive stress when the overload is relaxed.
The undesirable residual stresses created by bending to form parts (skin
tension caused by forming compression) are also mentioned.

Page 257 describes stress relieving via yielding.

Page 259 describes modifying residual stress by overloading:

"In springs, as in other parts that are primarily loaded in one
direction, an overload applied early in life is beneficial because it
introduces desirable residual compressive stresses at the proper
surface. Springs, hoists and pressure vessels are strengthened by proof
loading with a higher load than the expected service load"

see previous post.

existence of residual stress does not mean it causes spoke fatigue.
simple observation shows the truth. spokes are not observed to have
their cracking initiate in regions of high residual stress, but in
regions of high applied stress. as one might expect given that spoke
elbows, by definition, are subject to bending as a function of being
offset from the spoke axis.

simple observation of the facts. i suggest you try educating people on
basic scientific method rather than leaping to conclusions. or trolling.

"jim beam" wrote: existence of residual stress does not mean it causes
spoke fatigue.

Quoted message said:

simple observation shows the truth. spokes are not observed to have their
cracking initiate in regions of high residual stress, but in regions of
high applied stress. as one might expect given that spoke elbows, by
definition, are subject to bending as a function of being offset from the
spoke axis.

simple observation of the facts. i suggest you try educating people on
basic scientific method rather than leaping to conclusions. or trolling.

^^^^^^^^^^^^^^^^^^^^^^
Residual COMPRESSIVE stress would tend to reduce fatigue failure, since
fatigue cracks grow ONLY under tensile stress. That observation does not
depart from the scientific method. leap to conclusions, and certainly is not
trolling. Under tension, spoke bends try to straighten out, which creates
tensile stress on the inside of the bend. Any residual compressive stress
in that area as a result of the formation of the bend would have the effect
that Peter Cole referred to.

Leo Lichtman said:

"jim beam" wrote: existence of residual stress does not mean it causes
spoke fatigue.

Quoted message said:

simple observation shows the truth. spokes are not observed to have their
cracking initiate in regions of high residual stress, but in regions of
high applied stress. as one might expect given that spoke elbows, by
definition, are subject to bending as a function of being offset from the
spoke axis.

simple observation of the facts. i suggest you try educating people on
basic scientific method rather than leaping to conclusions. or trolling.

^^^^^^^^^^^^^^^^^^^^^^
Residual COMPRESSIVE stress would tend to reduce fatigue failure, since
fatigue cracks grow ONLY under tensile stress. That observation does not
depart from the scientific method. leap to conclusions, and certainly is not
trolling. Under tension, spoke bends try to straighten out, which creates
tensile stress on the inside of the bend. Any residual compressive stress
in that area as a result of the formation of the bend would have the effect
that Peter Cole referred to.

but the region of highest residual stress is near the neutral plane, not
the outer parts of the bend where fatigue is always observed to
initiate. and in fact, fatigue is still observed to initiate where, if
there is any, compressive residual is compressive, on the outer part of
the elbow.

again, observe the facts, bother to understand the whole story, and
don't leap to conclusions.

Leo Lichtman said:

"jim beam" wrote: existence of residual stress does not mean it causes
spoke fatigue.

Quoted message said:

simple observation shows the truth. spokes are not observed to have their
cracking initiate in regions of high residual stress, but in regions of
high applied stress. as one might expect given that spoke elbows, by
definition, are subject to bending as a function of being offset from the
spoke axis.

simple observation of the facts. i suggest you try educating people on
basic scientific method rather than leaping to conclusions. or trolling.

^^^^^^^^^^^^^^^^^^^^^^
Residual COMPRESSIVE stress would tend to reduce fatigue failure, since
fatigue cracks grow ONLY under tensile stress. That observation does not
depart from the scientific method. leap to conclusions, and certainly is not
trolling. Under tension, spoke bends try to straighten out, which creates
tensile stress on the inside of the bend. Any residual compressive stress
in that area as a result of the formation of the bend would have the effect
that Peter Cole referred to.

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.

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.

Peter Cole said:

Ben C said:

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

The point I made by posting the source was that overloading was a
recognized technique to manipulate residual stress -- either to reduce
or increase it depending on the desired outcome.

If a spoke is laced with an elbow angle that is too large, there will be
a bending stress in operation (load stress) that will put the outside
skin in tension. If the angle is too small, the load stress will be
tension on the inside skin. If the load path for a spoke is straight
from the hub to the rim, there will be no moment (bending stress), only
uniform tension across the cross section and shear stress.

By overloading the spoke, any existing notch conditions (small cracks,
threads) yield in tension and after unloading have residual compressive
stress which [censored] crack growth (see reference). The important factor
is that the static load plus overload plus residual totals to greater
than yield, if only in very local spots where stresses become naturally
concentrated.

As for the claim that spokes always crack from the outside of the elbow
(which doesn't agree with my limited experience), it's a certainty that
cold forming a ~90 degree bend will leave micro cracks on the outside
skin. Stress relief will yield these and generate beneficial
(compressive) residual stress in the immediate vicinity (see reference).
It does not matter if the residual skin stress from forming was
compressive, the stress relief will mitigate the fatigue effect of
surface flaws and provide additional benefit. As Jobst has frequently
pointed out, these effects are at the microscopic level, the source I
cited explains the mechanism.

Stress relief by brief overload before a part is put into service is a
well established method for improving fatigue life. The only requirement
is that the overload be applied in the same direction as the service
load. The literature abounds with examples, I just cited one source.
This can only be controversial via willful ignorance.

yet again, fatigue is NOT observed to be initiating in regions affected
by high residual stress, "relieved" or not. and it's independent of
whether any residual is either compressive or tensile. but it IS
observed to be originating in regions of high /applied/ stress. and
that high applied stress is entirely a function of the design of the
component.

if you want to fix spoke breakage, change the design - don't waste your
time clutching at straws demonstrating ignorance and inability to
observe. move to straight pull spokes. that's what the smart
manufacturers with research budgets and engineers that have done their
homework have done.

Peter Cole said:

Ben C said:

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

The point I made by posting the source was that overloading was a
recognized technique to manipulate residual stress -- either to reduce
or increase it depending on the desired outcome.

Interesting point and thank you for posting it. The idea of creating
residual compressive stress at a notch as you describe is not something
I've heard before.

Quoted message said:

If a spoke is laced with an elbow angle that is too large, there will be
a bending stress in operation (load stress) that will put the outside
skin in tension. If the angle is too small, the load stress will be
tension on the inside skin. If the load path for a spoke is straight
from the hub to the rim, there will be no moment (bending stress), only
uniform tension across the cross section and shear stress.

By overloading the spoke, any existing notch conditions (small cracks,
threads) yield in tension and after unloading have residual compressive
stress which [censored] crack growth (see reference). The important factor
is that the static load plus overload plus residual totals to greater
than yield, if only in very local spots where stresses become naturally
concentrated.

As for the claim that spokes always crack from the outside of the elbow
(which doesn't agree with my limited experience)

I don't know who's claiming that. My understanding was that if residual
stress were a factor, we would expect to see the majority of outbound
spoke failures starting from the inside and the majority of inbound
spokes failures starting from the outside.

Let me just check I got that the right way round... Yes I think so since
residual stress is tensile on the outside of the bend for a spoke whose
angle you made less acute (inbound), and the other way round for the
other ones.

The highest residual stresses I think jim beam has been saying are in
the interior of the spoke and not on the skin at all.

But, we don't see any particular pattern of whether failure starts on
the outside or inside, or on the exterior or in the interior, for
outbound or inbound spokes one way or the other.

But as I said we don't have much evidence that there isn't such a
pattern either, since most people don't look at their broken spokes in
such detail.

It's a pity Jobst didn't since he reports experiencing a big change in
number of broken spokes after he started stress-relieving. Examination
of the broken spokes might have helped confirm the theory that residual
stress was a significant factor in why they broke.

But I think you're saying with this new link that fatigue would be
mitigated at notches on either side of either kind of spoke anyway.

Quoted message said:

, it's a certainty that cold forming a ~90 degree bend will leave
micro cracks on the outside skin. Stress relief will yield these and
generate beneficial (compressive) residual stress in the immediate
vicinity (see reference). It does not matter if the residual skin
stress from forming was compressive, the stress relief will mitigate
the fatigue effect of surface flaws and provide additional benefit.

As Jobst has frequently
pointed out, these effects are at the microscopic level, the source I
cited explains the mechanism.

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

Quoted message said:

Stress relief by brief overload before a part is put into service is a
well established method for improving fatigue life. The only requirement
is that the overload be applied in the same direction as the service
load. The literature abounds with examples, I just cited one source.
This can only be controversial via willful ignorance.

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

Peter Cole said:

Ben C said:

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

The point I made by posting the source was that overloading was a
recognized technique to manipulate residual stress -- either to reduce
or increase it depending on the desired outcome.

If a spoke is laced with an elbow angle that is too large, there will be
a bending stress in operation (load stress) that will put the outside
skin in tension. If the angle is too small, the load stress will be
tension on the inside skin. If the load path for a spoke is straight
from the hub to the rim, there will be no moment (bending stress), only
uniform tension across the cross section and shear stress.

By overloading the spoke, any existing notch conditions (small cracks,
threads) yield in tension and after unloading have residual compressive
stress which [censored] crack growth (see reference). The important factor
is that the static load plus overload plus residual totals to greater
than yield, if only in very local spots where stresses become naturally
concentrated.

As for the claim that spokes always crack from the outside of the elbow
(which doesn't agree with my limited experience), it's a certainty that
cold forming a ~90 degree bend will leave micro cracks on the outside
skin. Stress relief will yield these and generate beneficial
(compressive) residual stress in the immediate vicinity (see reference).
It does not matter if the residual skin stress from forming was
compressive, the stress relief will mitigate the fatigue effect of
surface flaws and provide additional benefit. As Jobst has frequently
pointed out, these effects are at the microscopic level, the source I
cited explains the mechanism.

Stress relief by brief overload before a part is put into service is a
well established method for improving fatigue life. The only requirement
is that the overload be applied in the same direction as the service
load. The literature abounds with examples, I just cited one source.
This can only be controversial via willful ignorance.

There are other requirements than direction of applied proof load,
namely ductility and defect size. For a large defect in a ductile
material, you will in fact get plasticity and residual compression
when you release the load. If the defect is very small, or the
material is brittle, proof loading may form a crack. There will be
some residual compression at the crack tip, but not enough to make the
part stronger than it was before it was cracked.

Ben C said:

Peter Cole said:

Ben C wrote:

Quoted message said:

Quoted message said:

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

Quoted message said:

The point I made by posting the source was that overloading was a
recognized technique to manipulate residual stress -- either to reduce
or increase it depending on the desired outcome.

Interesting point and thank you for posting it. The idea of creating
residual compressive stress at a notch as you describe is not something
I've heard before.

Quoted message said:

If a spoke is laced with an elbow angle that is too large, there will be
a bending stress in operation (load stress) that will put the outside
skin in tension. If the angle is too small, the load stress will be
tension on the inside skin. If the load path for a spoke is straight
from the hub to the rim, there will be no moment (bending stress), only
uniform tension across the cross section and shear stress.

Quoted message said:

By overloading the spoke, any existing notch conditions (small cracks,
threads) yield in tension and after unloading have residual compressive
stress which [censored] crack growth (see reference). The important factor
is that the static load plus overload plus residual totals to greater
than yield, if only in very local spots where stresses become naturally
concentrated.

Quoted message said:

As for the claim that spokes always crack from the outside of the elbow
(which doesn't agree with my limited experience)

I don't know who's claiming that. My understanding was that if residual
stress were a factor, we would expect to see the majority of outbound
spoke failures starting from the inside and the majority of inbound
spokes failures starting from the outside.

Let me just check I got that the right way round... Yes I think so since
residual stress is tensile on the outside of the bend for a spoke whose
angle you made less acute (inbound), and the other way round for the
other ones.

The highest residual stresses I think jim beam has been saying are in
the interior of the spoke and not on the skin at all.

But, we don't see any particular pattern of whether failure starts on
the outside or inside, or on the exterior or in the interior, for
outbound or inbound spokes one way or the other.

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.

Quoted message said:

But as I said we don't have much evidence that there isn't such a
pattern either, since most people don't look at their broken spokes in
such detail.

It's a pity Jobst didn't since he reports experiencing a big change in
number of broken spokes after he started stress-relieving. Examination
of the broken spokes might have helped confirm the theory that residual
stress was a significant factor in why they broke.

But I think you're saying with this new link that fatigue would be
mitigated at notches on either side of either kind of spoke anyway.

Quoted message said:

, it's a certainty that cold forming a ~90 degree bend will leave
micro cracks on the outside skin. Stress relief will yield these and
generate beneficial (compressive) residual stress in the immediate
vicinity (see reference). It does not matter if the residual skin
stress from forming was compressive, the stress relief will mitigate
the fatigue effect of surface flaws and provide additional benefit.

Quoted message said:

As Jobst has frequently
pointed out, these effects are at the microscopic level, the source I
cited explains the mechanism.

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

Quoted message said:

Stress relief by brief overload before a part is put into service is a
well established method for improving fatigue life. The only requirement
is that the overload be applied in the same direction as the service
load. The literature abounds with examples, I just cited one source.
This can only be controversial via willful ignorance.

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

<[email hidden]> wrote:
This is because residual compression on one side of the bend is

Quoted message said:

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.

Ben C said:

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

Not true.

Quoted message said:

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.

Quoted message said:

There are other requirements than direction of applied proof load,
namely ductility and defect size. For a large defect in a ductile
material, you will in fact get plasticity and residual compression
when you release the load. If the defect is very small, or the
material is brittle, proof loading may form a crack. There will be
some residual compression at the crack tip, but not enough to make the
part stronger than it was before it was cracked.

That's not what my sources say. I'd be happy to look at yours.

Quoted message said:

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.

Ben C said:

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.

Quoted message said:

If the defect is very small, or the
material is brittle, proof loading may form a crack. There will be
some residual compression at the crack tip, but not enough to make the
part stronger than it was before it was cracked.

If you take the trouble to read the text I cited, you'll see the
specific case of gun barrels is described, where overloading serves 2
purposes -- either it will improve the fatigue life, or if the crack is
already severe it will cause a complete failure then and there. This is
exactly the principle Jobst Brandt describes in stress relieving spokes
that have already been in service.

Peter Cole said:

Ben C said:

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

Not true.

Quoted message said:

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.

Quoted message said:

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.

quote='Peter Cole'Ben C said:

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

Not true.

Quoted message said:

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.[/QUOTE]
"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.

On Apr 23, 4:11 pm, "Leo Lichtman" <[email hidden]>

Quoted message said:

<[email hidden]> 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.

I'm applying the equations for static equilibrium. If some part of
your spoke is under residual compression, the material around that
zone must be in tension to keep it there.

Peter Cole said:

Quoted message said:

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? 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.

In article <[email hidden]>,

daveornee said:

Peter Cole said:

Ben C said:

The controversy here is not that brief overload relieves stress or

that

Quoted message said:

stress relief improves fatigue life.

Not true.

Quoted message said:

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

Mavic

Quoted message said:

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.

Quoted message said:

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.

Quoted message said:

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.

Quoted message said:

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