Yet another broken spoke

[jim beam]

Ben C wrote:
> On 2007-09-08, jim beam <[email protected]> wrote:
>> Ben C wrote:
>>> On 2007-09-08, jim beam <[email protected]> wrote:
>>>> Ben C wrote:
>>>>> On 2007-09-07, Peter Cole <[email protected]> wrote:
>>>>> [...]
>>>>>> If the spoke has no bending moment (perfectly supported, perfect path),
>>>>>> the applied stress from spoke tension will be tensile (uniform) across
>>>>>> the cross section.
>>>>> I think I basically agree with this, though not absolutely. If the spoke
>>>>> is perfectly supported, but being pulled around a corner, there will
>>>>> surely still be a bending moment on parts of the spoke, but the distance
>>>>> component of that moment will never be greater than half the diameter of
>>>>> the spoke.
>>>>>
>>>>> Moments that small (assuming the force is in the range of normal spoke
>>>>> tensions) are low enough not to worry about-- they're not bringing
>>>>> anything anywhere near dangerously high stress levels for fatigue.
>>>> don't forget, virtually every fatigue failure there's ever been is
>>>> because of "unanticipated" factors. just because stress levels aren't
>>>> /thought/ to be high, *observed failures* tell us that there /is/
>>>> bending sufficient to cause fatigue!!!
>>> Yes, although doesn't it depend what you mean by "cause"? There is
>>> always some bending unless you have a straight pull spoke. If the mean
>>> stress in that bending cycle is low then you should get a long life
>>> unless you've got very bad surface defects.
>>>
>>> On the other hand if the mean stress is high (e.g. because you have a
>>> big bending moment), but the surface is much better, you might also get
>>> a short life.
>>>
>>> It's quite believable to me that wear resulting in corrosion that
>>> nucleated fatigue was a big factor in Clare's failed galvanized spokes,
>>> even if the stress cycle on them wasn't too big. But if someone's nice
>>> new stainless DT spokes in an Al hub fail after a few 100km then
>>> retained stress from the build or a big moment at the elbow look more to
>>> blame.
>> but this is not a matter of faith.
>>
>> 1. all traditional spokes of all qualities, of all materials, regardless
>> of manufacturer, are all have the elbow bent in a way that has the same
>> metallurgical result regarding residual stress. yet they all have
>> dramatically different fatigue lives that correspond with the other
>> variables such as material, surface finish, etc.
>
> Good point. I was careful though to say "retained stress from the
> build", not "residual stress" [from manufacture].
>
> The latter I have mostly put in the red herring bucket so far but keep
> an open mind as I am not an expert on these things.
>
> Retained stress from the build, which really I would lump together with
> poor spoke line (i.e. bending moment at the elbow) is what I suspect may
> be a significant factor in some failures where good quality spokes are
> used.
>
> If there is high stress in the elbow after the build it follows that
> there's enough moment present for spoke tension to maintain that stress.
> I'd be inclined to say that the moment is the real problem: it will
> allow a high-stress bending cycle in use, even if you stress-relieve, if
> stress relief doesn't also have the effect of also reducing that moment.
>
> But I think stress-relief probably _does_ reduce the moment by bending
> the elbow a bit more and by conforming it to the hub (_pace_ Peter
> Cole's and Jobst's well-reasoned arguments why hub conformance is not
> likely to happen much after tensioning).\

right, and that's distinct from "stress relief" in the metallurgical sense.

and yes, you /do/ get additional conformance beyond initial tensioning
by using overload - only if you don't understand point force deformation
you think otherwise.

 

[jim beam]

Ben C wrote:
> On 2007-09-09, [email protected] <[email protected]> wrote:
>> Ben C? writes:
> [...]
>>> Isn't it the other way round? The residual stresses are compressive
>>> on the exterior on outside of the bend (I thought) and tensile on
>>> the exterior of the inside. So it's the elbows that become more
>>> obtuse for which the building-bend adds to the manufacturing
>>> residual stress?
>> When the bend is made, the main stress is tension on the outside of
>> the bend
>
> Yes but that bend springs back, leaving residual compressive stress on
> the exterior on the outside?

yes indeed.


>
>> and that bend is increased when the outbound spokes get their
>> spoke line corrected, either by spoke tension or manually. In any
>> event, the outside of the spoke elbows are in tensile stress on those
>> spokes.
>
> Yes, the applied stress is on the outside of the bend on the exterior on
> the outbound spokes.

for heads-in spokes, yes indeed.


> [...]
>>> Yes, but as I said before, if the spoke is near yield under normal
>>> spoke tension _there must be a moment_. I don't see how you can
>>> have one without the other.
>> A spoke that is fully supported in the aluminum flange is loaded
>> purely in tension as it wraps around a curved bore that matches its
>> inner radius. The spoke head should rest solidly on the entrance to
>> the flange and the spoke pulled around the corner. Such a spoke will
>> last a long time. The straight pull spoke came along because people
>> were not seeing the effect of reasonable flange bores with reasonable
>> flange thickness and stress relieving.
>>
>>> That's why I think the moment is key: it's where big stresses at the
>>> elbow, on the exterior, come from.
>> There is no moment in a fully supported elbow and if you look at a
>> flange bore after removing the spoke you should see a smooth curve
>> that matches the spoke elbow shape.
>
> That's what one would hope for in a good wheel, certainly.
>
> But if there's no moment there's no stress there from the spoke line
> correction bend. Because there's nothing there to hold that bend in
> place.
>
> The last time we went through this it went slightly differently.
>
> Outbound spoke
> --------------
>
> 1. You said we bend the elbow to correct the line, it can't spring back,
> so the bend is held there, and so the stress remains high on the
> outside. I call this "retained build stress" to avoid confusion with
> manufacturing residual stress.
> 2. I said for spoke tension to hold the bend in place needs sufficient
> moment. If the spoke is flush to the flange you don't have that
> moment.
> 3. We repeated (1) and (2) to each other approximately 300 times.
> 4. Eventually you said yes but what do you mean by "flush"? It's never
> 100% completely totally and utterly flush. We get a little
> question-mark shaped hook, the details of which are complicated.
> 5. I said OK so there might be a moment, retained stress might be there,
> and stress-relief might get rid of it.
>
> But now you're saying "there's no moment". In that case there is no
> retained build stress on the outside of the bend. 100kgf or so _with no
> moment_ is not enough to hold a bend in a spoke.

yes indeed!


>
> Really they're the same thing. Residual stress (the kind that remains
> inside a wire after it's been bent and sprung back) requires knowledge
> of history. But retained build stress is just applied stress. It doesn't
> matter how it got there: if spoke tension is 100kgf and there's a
> too-big radius, you have a moment, and too-high stress on the elbow
> outside. No moment, nothing to worry about. If you stress-relieve and it
> makes the stress go away then it necessarily also made the moment go
> away.

how can it do that if the spoke is not a straight-pull spoke?


>
> So saying stress-relief is a process of helping things gain intimate
> conformity of some kind (as jim beam has said) and saying that it yields
> the bits of the cross section that are still close to yield from spoke
> line correction (as you have said) pretty much amount to the same thing.

the conformity to which /i/ refer is that of the hub hole. the spoke
hardly changes at all if it's not bent by the builder and the hub has
canted flanges.

http://www.flickr.com/photos/38636024@N00/331112190/


>
> As for residual manufacturing stress-- it sounds like that might add a
> bit to the tensile skin stress on the inside of the bend of the inbound
> spokes. So it may have a contribution to failures of inbound spokes that
> begin on the inside. But there's no reason to believe it's significant:
> Even if we accept the evidence that stress-relief practice reduces
> failures we don't know how much of that is to do with reducing retained
> build stress as opposed to reducing manufacturing residual stress.
>
> Since we're bending the elbow again anyway I'm inclined to think
> manufacturing residual stress is basically ancient history by the the
> time we've finished the wheel.

but jobst /invented/ the practice of "stress relief" and wants to sell
books based on it!!! [p.s., just overlook chronology issues with that
invention claim, ok?]

 

[jim beam]

Peter Cole wrote:
> jim beam wrote:
>> Peter Cole wrote:
>>> jim beam wrote:
>
>>>> you had to dig back /four years/ to find something that predated
>>>> luns tee reminding us all of residual stress profiles?
>>>
>>> I did a quick review of Luns' posts, I might have missed what you
>>> were referring to. Do any of these ring a bell (all responses to your
>>> posts)?
>>>
>>> " Quit your squirming." " What a load of hooey. "
>>> " Translation: you're pulling stuff out of thin air,"
>>> " I knew this would be your next dodge."
>
>>> Seems like he had a typical "jim beam" experience.
>>
>>
>> why don't you post from the jobstian library instead? his is much
>> more lengthy. you should try quoting that idiot krygowski some time
>> too. he's an engineering professor you know. or at least, it picks up
>> an engineering professor's paycheck.
>
> Your reference to Luns, not mine.
>
>>
>> as for luns, he's made some valuable contributions. but he's made
>> some mistakes too, most notably on the subject of strain aging
>> materials. he holds back a lot of stuff too - spoke tensiometer math
>> for example. but you don't quote that.
>
> Not relevant to the current thread.

whoops, not convenient!!! /so/ sorry.


> How do I quote something he "holds
> back" anyway? (just curious).
>
>
> why not? too intent on being a ***** to bother?
>
> Uh oh, nasty name time. Everybody knows what this means.

yes, it means that you're trying to wriggle, squirm and deceive and that
i'm calling you for what you are!

 

[jim beam]

Peter Cole wrote:
> jim beam wrote:
>> Peter Cole wrote:
>>> jim beam wrote:
>>>> Peter Cole wrote:
>>>>> jim beam wrote:
>>>>>
>>>>>> why don't /you/? but i forget, you don't contribute. maybe
>>>>>> that's a guilt thing since you're playing this game on your
>>>>>> employer's time, not your own.
>>>>>
>>>>> That's none of your business.
>>>>
>>>> it's a damned good indicator of your fundamental dishonesty though.
>>>
>>> That the best you got?
>>
>> there's more. but let's just focus on the facts.
>
> Then why introduce (ad hominem, no less) inventions?

invention? you post on your employer's time. that's no invention.

 

[Jambo]

"jim beam" <[email protected]> wrote in message
news:[email protected]...
> Jambo wrote:
>> Why are you such a hallucinating moron?
>>
>>
>
> are you missing your meds?

You didn't answer the question. Let me re-phrase:

Why are you such a delusional liar?

 

[Jambo]

"jim beam" <[email protected]> wrote in message
news:[email protected]...
> Jambo wrote:
>>
>> Hey dumbass, do you really think this sounds anywhere at all credible?
>> "softer and weaker" indeed, just like AL alloys having young's modulus a
>> magnitude less than CFRP, eh?
>
> er, look into cold work some time and learn what effect it has on hardness
> and strength. you /do/ know how to define "hardness" and "strength" don't
> you mr. modulus?

Hardness is your thick skull being impenetrable to real knowledge. Strength
is what's lacking in all your arguments. How's that?


> learn to use a killfile!

Just stay quiet, turdball.

 

[Peter Cole]

jim beam wrote:
> Peter Cole wrote:
>> jim beam wrote:
>>> Ben C wrote:
>>>> On 2007-09-07, [email protected]
>>>> <[email protected]> wrote:
>>>>> Ben C? writes:
>>>> [...]
>>>>>> MP Since that location has tensile residual stress, tensile applied
>>>>>> MP mean stress from the spoke tension and bending,
>>>>>> Is the _applied_ stress on the inside of the elbow from spoke
>>>>>> tension and bending really tensile?
>>>> [...]
>>>>>> I don't understand that. I thought when you bent a wire you got
>>>>>> tensile
>>>>>> stress on the outside of the bend and compressive on the inside?
>>>>> These loads tend to open the elbow angle so that causes tensile
>>>>> stress.
>>>>
>>>> Just to recap, because I thought this was (roughly) the picture:
>>>>
>>>> 1. I put an outbound spoke in. Its natural elbow angle is a bit too
>>>> wide.
>>>> 2. I tighten it up, the elbow bends a bit, making the elbow angle
>>>> smaller. It wants to spring back, but it can't, because it's
>>>> installed in the wheel and held in place.
>>>> 3. This leaves applied stress that's tensile on the outside of the
>>>> elbow
>>>> and compressive on the inside.
>>>> 4. Momentary overload and relaxation leaves a spoke with reduced
>>>> stresses.
>>>>
>>>> Do I have this (fundamentally) wrong?
>>>>
>>>> Perhaps the point is it's the other way around for an inbound spoke,
>>>> whose elbow gets opened a bit by being installed in the wheel.
>>>>
>>>>> As Mike mentioned above, springback makes the stress reverse
>>>>> from that during forming.
>>>>
>>>> Yes, I think I understand that part. That's residual stress from spoke
>>>> forming, not retained stress from wheel-building, as I understand it.
>>>> During wheelbuilding the spoke is not able to spring back, so an
>>>> outbound spoke remains in tensile stress on the outside and compressive
>>>> on the inside until you stress-relieve.
>>>
>>> only parts of it. read this from luns tee:
>>> http://groups.google.com/group/rec.bicycles.tech/msg/af080b93a59cca03
>>>
>>> most notably:
>>> "For a more severely bent wire, the yielded layers extend deeper,
>>> and the residual stress pattern becomes more like:
>>>
>>> cccTCttt "
>>>
>>> so here's the problem - that [simplified but useful] depiction shows
>>> where the residual stress profiles would be. if residual stress were
>>> causing fatigue, we would observe fatigue initiating at a "T" point.
>>> instead, we observe it initiating at /both/ "c"'s and "t"'s.
>>>
>>> "engineers" can argue all they want about what they think should be
>>> happening, but if observed facts tell a different story, it's just so
>>> much hot air.
>>
>> I performed the experiment Luns suggested on the above thread and
>> posted my results:
>>
>> http://tinyurl.com/356ru7
>>
>> I think that was an "observed fact". Yours?
>
> and your explanation of why spoke fatigue initiates at a region of
> little or zero residual stress is???
>
> http://www.flickr.com/photos/38636024@N00/1346747861/

Who says there's little or no residual stress at the surface? That's not
what I found when I did the experiment.

>>>
>>>>
>>>> After stress-relieving, the stresses may be the other way round again,
>>>> but more importantly, reduced in magnitude.
>>>>
>>>> It seems that residual stress from forming would be mitigated and/or
>>>> dwarfed in magnitude by retained applied stress from the build? So
>>>> perhaps residual stress from forming _is_ a red herring?
>>>
>>> truth is, outside of the lab and in carefully controlled
>>> environments, fatigue is *always* observed to initiate at surface
>>> defects. these can be from processing, corrosion, or even inclusions
>>> within the material. addressing each of these is observed to directly
>>> affect fatigue life.
>>> among these, electron microscopy shows inclusion content to be a
>>> significant fatigue initiator. removing inclusions is _proven_ to
>>> extend fatigue life considerably.
>>
>> Everybody knows this stuff. Lots of us have had nicked spokes break in
>> mid-span. So what? Stress + flaw = failure. Film at 11.
>
> eh? surface nicks are /not/ inclusions!!!

Both are defects (obviously).

>
>>
>>>
>>> that's why spoke manufacturers spend lots of money on expensive
>>> vacuum degassed materials. if cheap materials could offer superior
>>> fatigue life by way of simple stress relief, you'd better believe
>>> they'd be used.
>>
>> Vacuum degassing was big news in the 50's. It's a cheap bulk process,
>> common as dirt. What else have you got?
>
> it's /cheaper/ than it was, but it's still expensive.

No, it's not.

> and it didn't
> start being used for bike spoke material until the 70's

Cite, please. It was used in auto sheet metal by that time.

 

[Peter Cole]

jim beam wrote:
> Peter Cole wrote:
>> jim beam wrote:
>>> Peter Cole wrote:
>>>> jim beam wrote:
>>
>>>>> you had to dig back /four years/ to find something that predated
>>>>> luns tee reminding us all of residual stress profiles?
>>>>
>>>> I did a quick review of Luns' posts, I might have missed what you
>>>> were referring to. Do any of these ring a bell (all responses to
>>>> your posts)?
>>>>
>>>> " Quit your squirming." " What a load of hooey. "
>>>> " Translation: you're pulling stuff out of thin air,"
>>>> " I knew this would be your next dodge."
>>
>>>> Seems like he had a typical "jim beam" experience.
>>>
>>>
>>> why don't you post from the jobstian library instead? his is much
>>> more lengthy. you should try quoting that idiot krygowski some time
>>> too. he's an engineering professor you know. or at least, it picks
>>> up an engineering professor's paycheck.
>>
>> Your reference to Luns, not mine.
>>
>>>
>>> as for luns, he's made some valuable contributions. but he's made
>>> some mistakes too, most notably on the subject of strain aging
>>> materials. he holds back a lot of stuff too - spoke tensiometer math
>>> for example. but you don't quote that.
>>
>> Not relevant to the current thread.
>
> whoops, not convenient!!! /so/ sorry.

OT, in other words (your specialty).


>
>
>> How do I quote something he "holds back" anyway? (just curious).
>>
>>
>> why not? too intent on being a ***** to bother?
>>
>> Uh oh, nasty name time. Everybody knows what this means.
>
> yes, it means that you're trying to wriggle, squirm and deceive and that
> i'm calling you for what you are!

No, it means you've run out of argument. You're nothing if not predictable.

 

[Peter Cole]

jim beam wrote:
> Peter Cole wrote:
>> jim beam wrote:
>>> Peter Cole wrote:
>>>> jim beam wrote:
>>>>> Peter Cole wrote:
>>>>>> jim beam wrote:
>>>>>>
>>>>>>> why don't /you/? but i forget, you don't contribute. maybe
>>>>>>> that's a guilt thing since you're playing this game on your
>>>>>>> employer's time, not your own.
>>>>>>
>>>>>> That's none of your business.
>>>>>
>>>>> it's a damned good indicator of your fundamental dishonesty though.
>>>>
>>>> That the best you got?
>>>
>>> there's more. but let's just focus on the facts.
>>
>> Then why introduce (ad hominem, no less) inventions?
>
> invention? you post on your employer's time. that's no invention.

I'm sure if you think *real hard* you can come up with another
explanation. Or maybe not.

 

[clare at snyder.on.ca]

On Fri, 07 Sep 2007 21:07:51 -0700, jim beam
<[email protected]> wrote:

>clare at snyder.on.ca wrote:
>> All this talk about "stress relieving" spokes - I thought "stress
>> relieving" was a heat treatment. Dig out the Bernzo-Matic and heat 'em
>> up cherry red. Quench. Heat to (straw or whatever temper temperature)
>> and let air cool. NOW the spokes are stress relieved.
>
>they are, but they're also now softer and weaker - not strong enough for
>their application. that's why manufacturers don't do this.


No, properly done the spokes would be hard and stronger. (if the right
steel was used to start with)

--
Posted via a free Usenet account from http://www.teranews.com

 

[Ben C]

On 2007-09-09, jim beam <[email protected]> wrote:
> Ben C wrote:
[...]
>> I'm not denying that residual stresses from manufacture will add to
>> those stresses in places.
>
> but they're not observed to be where fatigue initiates!

I gather from discussions that the regions of highest residual stress
are on the interior of the material, where fatigue doesn't initiate
usually for spokes.

But there is some residual stress on the exterior as well right?

Some of that might be expected to actually mitigate fatigue. But we do
have some tensile residual stress on the exterior of the inside of the
inbound spokes?

"Yes but it's a herring" or "No but it's a herring" are both acceptable
answers

 

[Ben C]

On 2007-09-09, jim beam <[email protected]> wrote:
> Ben C wrote:
[...]
>> Really they're the same thing. Residual stress (the kind that remains
>> inside a wire after it's been bent and sprung back) requires knowledge
>> of history. But retained build stress is just applied stress. It doesn't
>> matter how it got there: if spoke tension is 100kgf and there's a
>> too-big radius, you have a moment, and too-high stress on the elbow
>> outside. No moment, nothing to worry about. If you stress-relieve and it
>> makes the stress go away then it necessarily also made the moment go
>> away.
>
> how can it do that if the spoke is not a straight-pull spoke?

Correct, it can't. The moment never goes away completely, but if it's
small enough, stresses in use are low enough that fatigue life should be
long.

>> So saying stress-relief is a process of helping things gain intimate
>> conformity of some kind (as jim beam has said) and saying that it yields
>> the bits of the cross section that are still close to yield from spoke
>> line correction (as you have said) pretty much amount to the same thing.
>
> the conformity to which /i/ refer is that of the hub hole. the spoke
> hardly changes at all if it's not bent by the builder and the hub has
> canted flanges.

A bit of both I think. When the moment is small I think the hub may
yield more as it gets harder and harder to bend the spoke.

I know it gets harder to sink wire into aluminium the deeper you go, but
I also think things are moving around and the spoke may therefore be
ploughing into some previously unvisited areas of aluminium during
"stress-relief".

But I honestly don't know.

> http://www.flickr.com/photos/38636024@N00/331112190/

But depends on the components, who's building it, etc. Spokes _can_ get
more bent than that even if you don't thumb them.

 

[Michael Press]

In article
<[email protected]>,
Ben C <[email protected]> wrote:

> It seems that residual stress from forming would be mitigated and/or
> dwarfed in magnitude by retained applied stress from the build? So
> perhaps residual stress from forming _is_ a red herring?

Retained stress from forming _plus_ applied stress from
tension in the wheel can bring some portions of the
spoke to the edge of yield. Cyclic unloading and
loading of the spoke during use can initiate fatigue
cracks.

A spoke elbow is cold formed by wrapping it around a
tool and bringing it to yield. In use an outbound spoke
elbow is wrapped around the hub flange, simulating the
tool upon which it was formed, bringing it back near
yield again. Stressing the spoke in a built up
tensioned wheel takes the spoke past the yield point.
Now in use it will not get near the yield point again.

--
Michael Press

 

[Michael Press]

In article
<[email protected]>,
Peter Cole <[email protected]> wrote:

> jim beam wrote:
> > Peter Cole wrote:
> >> jim beam wrote:
>
> >>> truth of the saying, "ignorance can be cured, stupid is forever".
> >>
> >> Just out of curiosity, I Googled to see where this was originally
> >> explained to you.
> >>
> >> It was over 4 years ago. I thought Mike Prime (a metallurgist) did a
> >> good job. Apparently it didn't stick. I can see why Jobst no longer
> >> bothers to respond.
> >>
> >> http://tinyurl.com/29v4u2
> >
> > you had to dig back /four years/ to find something that predated luns
> > tee reminding us all of residual stress profiles?
>
> I did a quick review of Luns' posts, I might have missed what you were
> referring to. Do any of these ring a bell (all responses to your posts)?
>
> " Quit your squirming. You change your tune every time I point
> out how something in what you've said is impossible. This has nothing to
> do with net stress across the spoke, this is about the net effect of
> all applied forces at the inner surface of the spoke where you seem to
> believe a neutral axis exists. Besides, what you point out here also
> contradicts your claim of the neutral axis being at the surface: zero
> tensile/compressive stress (definition of neutral axis) at the
> surface, and tension through the whole spoke outside of that, is not a
> net of zero across the spoke."
>
> -Luns

So, jb thinks that a spoke elbow with residual stresses
can have a neutral axis at the surface. Priceless.

--
Michael Press

 

[[email protected]]

On Sun, 09 Sep 2007 02:55:50 -0500, Ben C <[email protected]> wrote:

[snip]

>If a spoke isn't stress relieved, and run at a high mean stress stress
>cycle for some time, and then stress-relieved just in the nick of time
>before it actually breaks, what is its life time from that point
>expected to be? In theory?
>
>Not rhetorical question.

[snip]

Dear Ben,

Unfortunately, only theoretical answers are available.

The enormous fuss in these threads is due to the fact that we know of
no spoke fatigue tests except for the one done in 1985 for Wheelsmith
at Stanford, which did not address the various theories frequently
offered here.

Everything else is based on anecdotes.

For the curious:

"In 1984 and 1985, fatigue tests on stainless steel bicycle spokes
were carried out for Wheelsmith, Inc. at Stanford University. Constant
cycle tests were conducted with pre-tensioning stresses of 174 MPa,
250 MPa, 343 MPa, 347 MPa, 424 MPa, and 501 MPa. Any correlation
between the cycles to failure, N, and the pre-tensioning stress was
obscured by random variations in N. The parameters a and b were
determined from a least squares fit of fatigue data from 76 stainless
steel bicycle spokes, shown in Figure 13 (using N as the dependent
variable (Rice 1985)). Variations in the (dependent) variable N due to
stress cycles of a constant amplitude, S, are modeled by the normally
distributed random variable b, with average b, and coefficient of
variation Vb. For the data shown in Figure 13, a=-0.30
log(MPa)/log(cycle), b =4.12 log(MPa), Vb=0.017. Using this
data, the coefficient of variation of N is 0.74, for any constant
value of S. This data is unique to the spoke material, the spoke
geometry, and the spoke’s particular stress concentrations. In 68
spokes the failure occurred at the cold-worked elbow; in the remaining
8 spokes the failure occurred at the threads."

"The smallest stress cycle in the fatigue tests was 174 MPa, whereas
the stress range from the road test data was 20 MPa to 150 MPa. Hence,
the fatigue data was extrapolated to the low stress range. (To have
tested a single sample at 40 MPa would have required over a year of
continuous testing at 10 cycles per second, and an unwarranted use of
facilities.)"

http://www.duke.edu/~hpgavin/papers/HPGavin-Wheel-Paper.pdf

Cheers,

Carl Fogel

 

[[email protected]]

Ben C? writes:

>> After replacing the broken spokes and fearing that others would
>> soon fail, I "stress relieved" the wheels, breaking two more spokes
>> that I then replaced before repeating the spoke breaking exercise.
>> It occurred to me weeks later that no more spokes had broken and
>> that the wheels were performing reliably.

> If a spoke isn't stress relieved, and run at a high mean stress
> stress cycle for some time, and then stress-relieved just in the
> nick of time before it actually breaks, what is its life time from
> that point expected to be? In theory?

Your assumption is that other spokes in the wheel all have high stress
somewhere and that they are all progressing to failure uniformly.
This is especially untrue for wheels that have relatively early
failures. My experience is that even with onset of fatigue cracking,
stress relieving and replacement of failed spokes is worth the effort.

>>> Two reasons were given why loose spokes might lead to failure:
>>> wear and bending.

>> From the evidence reported, I do not understand the bending theory
>> of loose spokes. Bending under essentially no load is like holding
>> a spoke by one end and waving it back and forth manually.

> The rim is pushing the spoke toward the hub. First it (a) wobbles
> a bit, and then, (b) not being a stiff rod, it may flex one way or
> another. (c) It's supported at the elbow so it might bend there.

> jim beam has also suggested the spoke crossings may be involved
> although I'm less clear on the details so won't try to explain that.

Having not seen a spoke fail at a spoke crossing, this does not ring
true. If you believe the spoke crossings bend spokes at the elbows,
this runs into a counter claim by that author that flexing is not
affected by spoke preload.

> (b) may not happen depending on the magnitude of (a)-- I believe
> that's what you said. So I don't know if (b) or (c) happens.

> That's all there is to it. That's all the reasoning I know of.
> There's no evidence.

In that event, I see no reason to give that credibility in light of my
experience with riders who had loose spokes in the rear wheel to the
extent that they rattled when standing, without failure and that when
subsequently re-trued and tightened served well... in the days of poor
quality spokes.

>>> And yes, fair point, we haven't actually seen the worn galvanized
>>> spokes but Clare provided a convincing description of them. I
>>> think Gene may have reported similar problems in the past and has
>>> also described movement in the spoke holes of loose spokes leading
>>> to wear.

>> I don't believe that they were worn half way through or even more
>> than polishing the plating.

> Isn't a galvanized spoke coated with a kind of zinc stuff to stop it
> rusting? If that wears away the steel underneath starts to rust a
> bit. A small rust spot is enough to nucleate fatigue. So the spoke
> doesn't wear all the way through, or rust all the way through, but a
> bit of wear is enough to make it fatigue quickly.

Usually zinc plating (spokes are not galvanized in molten zinc as
sheet metal often is) zinc usually has a matte finish and becomes
polished under contact loads with an aluminum flange. Steel hubs
present different problems, but I don't believe we are discussing
steel hubs in this thread.

Jobst Brandt

 

[[email protected]]

Ben C? writes:

>>> Isn't it the other way round? The residual stresses are
>>> compressive on the exterior on outside of the bend (I thought) and
>>> tensile on the exterior of the inside. So it's the elbows that
>>> become more obtuse for which the building-bend adds to the
>>> manufacturing residual stress?

>> When the bend is made, the main stress is tension on the outside of
>> the bend

> Yes but that bend springs back, leaving residual compressive stress
> on the exterior on the outside?

>> and that bend is increased when the outbound spokes get their spoke
>> line corrected, either by spoke tension or manually. In any event,
>> the outside of the spoke elbows are in tensile stress on those
>> spokes.

Increasing the bend brings it back to the prior condition where it had
tensile stress on the outside of the bend and increases that bend. If
it is a slight bend, then it may leave the outside of the elbow
neutral, but don't bet on it. If you unspoke a used wheel, you'll
note that the outbound spokes have a greater permanent bend than new
spokes, showing that they were brought to yield in the process.

> Yes, the applied stress is on the outside of the bend on the
> exterior on the outbound spokes.

>>> Yes, but as I said before, if the spoke is near yield under normal
>>> spoke tension _there must be a moment_. I don't see how you can
>>> have one without the other.

>> A spoke that is fully supported in the aluminum flange is loaded
>> purely in tension as it wraps around a curved bore that matches its
>> inner radius. The spoke head should rest solidly on the entrance
>> to the flange and the spoke pulled around the corner. Such a spoke
>> will last a long time. The straight pull spoke came along because
>> people were not seeing the effect of reasonable flange bores with
>> reasonable flange thickness and stress relieving.

>>> That's why I think the moment is key: it's where big stresses at the
>>> elbow, on the exterior, come from.

>> There is no moment in a fully supported elbow and if you look at a
>> flange bore after removing the spoke you should see a smooth curve
>> that matches the spoke elbow shape.

> That's what one would hope for in a good wheel, certainly.

Even in a less than perfect wheel build, tensile loads on a freshly
built wheel that has been stress relieved has higher stress on the
outside of the bend than on the inside, just by geometry.

> But if there's no moment there's no stress there from the spoke line
> correction bend. Because there's nothing there to hold that bend in
> place.

> The last time we went through this it went slightly differently.

Moment implied that the spoke is loaded in bending at that point, but
even a spoke that has not been stress relieved, the spoke is generally
fully supported in the aluminum of a flange as the aluminum yields.
Your "moment" seems to me to be undefined. I don't see a moment
unless the elbow is excessively long as they were for a while from DT
causing many to break.

> Outbound spoke
> --------------

> 1. You said we bend the elbow to correct the line, it can't spring
> back, so the bend is held there, and so the stress remains high
> on the outside. I call this "retained build stress" to avoid
> confusion with manufacturing residual stress.

> 2. I said for spoke tension to hold the bend in place needs
> sufficient moment. If the spoke is flush to the flange you don't
> have that moment.

> 3. We repeated (1) and (2) to each other approximately 300 times.

> 4. Eventually you said yes but what do you mean by "flush"? It's
> never 100% completely totally and utterly flush. We get a little
> question-mark shaped hook, the details of which are complicated.

> 5. I said OK so there might be a moment, retained stress might be
> there, and stress-relief might get rid of it.

> But now you're saying "there's no moment". In that case there is no
> retained build stress on the outside of the bend. 100kgf or so
> _with no moment_ is not enough to hold a bend in a spoke.

If you insist and referring to the stress in the elbow as a moment
then you are dodging the stress at that point which is the operating
effect that causes failure. If that stress can be reduced through
temporary overtightening (stress relief) than I don't see how calling
it a moment properly defines the condition.

> Really they're the same thing. Residual stress (the kind that
> remains inside a wire after it's been bent and sprung back) requires
> knowledge of history. But retained build stress is just applied
> stress. It doesn't matter how it got there: if spoke tension is
> 100kgf and there's a too-big radius, you have a moment, and too-high
> stress on the elbow outside. No moment, nothing to worry about. If
> you stress-relieve and it makes the stress go away then it
> necessarily also made the moment go away.

As I said, you'll find that spokes generally are supported throughout
the elbow bend by a yielded aluminum flange. The moment is not the
\operator here but the stress that remains in the elbow from
manufacture and build.

> So saying stress-relief is a process of helping things gain intimate
> conformity of some kind (as jim beam has said) and saying that it
> yields the bits of the cross section that are still close to yield
> from spoke line correction (as you have said) pretty much amount to
> the same thing.

I didn't say that. That is the claim by the former metallurgist who
calls it bedding-in or setting.

> As for residual manufacturing stress-- it sounds like that might add
> a bit to the tensile skin stress on the inside of the bend of the
> inbound spokes. So it may have a contribution to failures of
> inbound spokes that begin on the inside. But there's no reason to
> believe it's significant: Even if we accept the evidence that
> stress-relief practice reduces failures we don't know how much of
> that is to do with reducing retained build stress as opposed to
> reducing manufacturing residual stress.

I think I covered that and it is not as you say.

> Since we're bending the elbow again anyway I'm inclined to think
> manufacturing residual stress is basically ancient history by the
> the time we've finished the wheel.

It assures tensile stress on the outside of the elbow and in the
threads.

Jobst Brandt

 

[jim beam]

Michael Press wrote:
> In article
> <[email protected]>,
> Peter Cole <[email protected]> wrote:
>
>> jim beam wrote:
>>> Peter Cole wrote:
>>>> jim beam wrote:
>>>>> truth of the saying, "ignorance can be cured, stupid is forever".
>>>> Just out of curiosity, I Googled to see where this was originally
>>>> explained to you.
>>>>
>>>> It was over 4 years ago. I thought Mike Prime (a metallurgist) did a
>>>> good job. Apparently it didn't stick. I can see why Jobst no longer
>>>> bothers to respond.
>>>>
>>>> http://tinyurl.com/29v4u2
>>> you had to dig back /four years/ to find something that predated luns
>>> tee reminding us all of residual stress profiles?
>> I did a quick review of Luns' posts, I might have missed what you were
>> referring to. Do any of these ring a bell (all responses to your posts)?
>>
>> " Quit your squirming. You change your tune every time I point
>> out how something in what you've said is impossible. This has nothing to
>> do with net stress across the spoke, this is about the net effect of
>> all applied forces at the inner surface of the spoke where you seem to
>> believe a neutral axis exists. Besides, what you point out here also
>> contradicts your claim of the neutral axis being at the surface: zero
>> tensile/compressive stress (definition of neutral axis) at the
>> surface, and tension through the whole spoke outside of that, is not a
>> net of zero across the spoke."
>>
>> -Luns
>
> So, jb thinks that a spoke elbow with residual stresses
> can have a neutral axis at the surface. Priceless.
>

don't put those [untrue] words in my mouth.

and you might benefit from re-reading what i actually /did/ say.

 

[jim beam]

Peter Cole wrote:
> jim beam wrote:
>> Peter Cole wrote:
>>> jim beam wrote:
>>>> Ben C wrote:
>>>>> On 2007-09-07, [email protected]
>>>>> <[email protected]> wrote:
>>>>>> Ben C? writes:
>>>>> [...]
>>>>>>> MP Since that location has tensile residual stress, tensile applied
>>>>>>> MP mean stress from the spoke tension and bending,
>>>>>>> Is the _applied_ stress on the inside of the elbow from spoke
>>>>>>> tension and bending really tensile?
>>>>> [...]
>>>>>>> I don't understand that. I thought when you bent a wire you got
>>>>>>> tensile
>>>>>>> stress on the outside of the bend and compressive on the inside?
>>>>>> These loads tend to open the elbow angle so that causes tensile
>>>>>> stress.
>>>>>
>>>>> Just to recap, because I thought this was (roughly) the picture:
>>>>>
>>>>> 1. I put an outbound spoke in. Its natural elbow angle is a bit too
>>>>> wide.
>>>>> 2. I tighten it up, the elbow bends a bit, making the elbow angle
>>>>> smaller. It wants to spring back, but it can't, because it's
>>>>> installed in the wheel and held in place.
>>>>> 3. This leaves applied stress that's tensile on the outside of the
>>>>> elbow
>>>>> and compressive on the inside.
>>>>> 4. Momentary overload and relaxation leaves a spoke with reduced
>>>>> stresses.
>>>>>
>>>>> Do I have this (fundamentally) wrong?
>>>>>
>>>>> Perhaps the point is it's the other way around for an inbound spoke,
>>>>> whose elbow gets opened a bit by being installed in the wheel.
>>>>>
>>>>>> As Mike mentioned above, springback makes the stress reverse
>>>>>> from that during forming.
>>>>>
>>>>> Yes, I think I understand that part. That's residual stress from spoke
>>>>> forming, not retained stress from wheel-building, as I understand it.
>>>>> During wheelbuilding the spoke is not able to spring back, so an
>>>>> outbound spoke remains in tensile stress on the outside and
>>>>> compressive
>>>>> on the inside until you stress-relieve.
>>>>
>>>> only parts of it. read this from luns tee:
>>>> http://groups.google.com/group/rec.bicycles.tech/msg/af080b93a59cca03
>>>>
>>>> most notably:
>>>> "For a more severely bent wire, the yielded layers extend deeper,
>>>> and the residual stress pattern becomes more like:
>>>>
>>>> cccTCttt "
>>>>
>>>> so here's the problem - that [simplified but useful] depiction shows
>>>> where the residual stress profiles would be. if residual stress
>>>> were causing fatigue, we would observe fatigue initiating at a "T"
>>>> point. instead, we observe it initiating at /both/ "c"'s and "t"'s.
>>>>
>>>> "engineers" can argue all they want about what they think should be
>>>> happening, but if observed facts tell a different story, it's just
>>>> so much hot air.
>>>
>>> I performed the experiment Luns suggested on the above thread and
>>> posted my results:
>>>
>>> http://tinyurl.com/356ru7
>>>
>>> I think that was an "observed fact". Yours?
>>
>> and your explanation of why spoke fatigue initiates at a region of
>> little or zero residual stress is???
>>
>> http://www.flickr.com/photos/38636024@N00/1346747861/
>
> Who says there's little or no residual stress at the surface? That's not
> what I found when I did the experiment.

but you did! you obviously didn't understand what you were observing.


>
>>>>
>>>>>
>>>>> After stress-relieving, the stresses may be the other way round again,
>>>>> but more importantly, reduced in magnitude.
>>>>>
>>>>> It seems that residual stress from forming would be mitigated and/or
>>>>> dwarfed in magnitude by retained applied stress from the build? So
>>>>> perhaps residual stress from forming _is_ a red herring?
>>>>
>>>> truth is, outside of the lab and in carefully controlled
>>>> environments, fatigue is *always* observed to initiate at surface
>>>> defects. these can be from processing, corrosion, or even
>>>> inclusions within the material. addressing each of these is observed
>>>> to directly affect fatigue life.
>>>> among these, electron microscopy shows inclusion content to be a
>>>> significant fatigue initiator. removing inclusions is _proven_ to
>>>> extend fatigue life considerably.
>>>
>>> Everybody knows this stuff. Lots of us have had nicked spokes break
>>> in mid-span. So what? Stress + flaw = failure. Film at 11.
>>
>> eh? surface nicks are /not/ inclusions!!!
>
> Both are defects (obviously).

wriggle, squirm. a nick is not an inclusion. period.

>
>>
>>>
>>>>
>>>> that's why spoke manufacturers spend lots of money on expensive
>>>> vacuum degassed materials. if cheap materials could offer superior
>>>> fatigue life by way of simple stress relief, you'd better believe
>>>> they'd be used.
>>>
>>> Vacuum degassing was big news in the 50's. It's a cheap bulk process,
>>> common as dirt. What else have you got?
>>
>> it's /cheaper/ than it was, but it's still expensive.
>
> No, it's not.

er, it is actually.


>
>> and it didn't
>> start being used for bike spoke material until the 70's
>
> Cite, please. It was used in auto sheet metal by that time.

not even in the 80's big guy. that's the last time i went through a
strip mill and it was either open ingot or con-cast. look at this stuff
under a microscope some time and you'll see the evidence for yourself.

 

[jim beam]

Peter Cole wrote:
> jim beam wrote:
>> Peter Cole wrote:
>>> jim beam wrote:
>>>> Peter Cole wrote:
>>>>> jim beam wrote:
>>>>>> Peter Cole wrote:
>>>>>>> jim beam wrote:
>>>>>>>
>>>>>>>> why don't /you/? but i forget, you don't contribute. maybe
>>>>>>>> that's a guilt thing since you're playing this game on your
>>>>>>>> employer's time, not your own.
>>>>>>>
>>>>>>> That's none of your business.
>>>>>>
>>>>>> it's a damned good indicator of your fundamental dishonesty though.
>>>>>
>>>>> That the best you got?
>>>>
>>>> there's more. but let's just focus on the facts.
>>>
>>> Then why introduce (ad hominem, no less) inventions?
>>
>> invention? you post on your employer's time. that's no invention.
>
> I'm sure if you think *real hard* you can come up with another
> explanation. Or maybe not.

you're unemployed? got competency issues?