Yet another broken spoke

[jim beam]

Ben C wrote:
> 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

yes, there can be a small residual stress at the initiation point, but
if we _observe_ fatigue initiating at a point where there is supposed to
be /compressive/ residual stress, and we sometimes do, then clearly
residual stress is a great big red herring.

 

[jim beam]

clare at snyder.on.ca wrote:
> 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)
>
there are no heat treated spokes that i know of - all strengthening is
the result of cold work, and thermal stress relief would reduce that.

 

[jim beam]

Michael Press wrote:
> 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.

that can't be what we find in spokes though because if it's at or even
near yield, fatigue life is minimal.


> 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.

 

[jim beam]

[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?
>
>>> 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.

what? like these spokes?
http://flickr.com/photos/38636024@N00/331112190/


>
>> 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.

unless the hub flange is canted. which most are these days.


>
>> 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.

not against axial loading its not. and that's what we need to worry about!

> 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.

see above.


>
>> 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.

against one direction of lateral loading, but not axial loading or the
other direction of lateral which may be caused by an interleaving partner.


>
>> 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.

er, /you/ are the spoke line "correction" guy, not me.


>
>> 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.

so post some numbers then! if your assertion is true, quantify it
scientifically and publish!!!

in the mean time, the rest of us will examine actual fatigue failures
and note the inconsistency between your theory and reality - on many levels.


>
>> 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.

manufacturing residual stress is supposed to be tensile on the outside
of the bend? please clarify your position on this one. same for
threads - the thread root particularly.

 

[jim beam]

[email protected] wrote:
> 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.

quantifiably, right? you have numbers?


>
>>>> 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.

that's not what i'm saying at all. what i /am/ saying is that if a
spoke is interleaved, if it goes slack, the tension from its partner
will cause considerably more bending excursion than if it had not been
interleaved.


>
>> (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.

number please...



>
>>>> 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)

the process is called "sherardizing"


> zinc usually has a matte finish and becomes
> polished under contact loads with an aluminum flange.

which implies relative movement. which you say is impossible if the
spoke is supported by the flange!

oh, and zinc protects steel by preferential corrosion. that means it
more aggressively corrodes relative to its substrate - it doesn't stay
polished. hence the surface will become rougher, and more fatigue
prone. less so than plain steel obviously, but more so than stainless.

> Steel hubs
> present different problems, but I don't believe we are discussing
> steel hubs in this thread.

red herring.

 

[Ben C]

On 2007-09-10, [email protected] <[email protected]> wrote:
> Ben C? writes:
[...]
>>> 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.

I don't think the suggestion is that they fail at the crossings, but
that the way the crossings move allow them to flex at the elbows.

 

[Ben C]

On 2007-09-10, [email protected] <[email protected]> wrote:
[...]
>> 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.

Well, I've explained it as clearly as I can and I don't believe it's
that difficult to understand.

[...]
>> 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.

Well obviously.

>> 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.

I did write that rather obliquely. What I meant to say is that the fact
that parts of the cross-section are yielded by spoke line correction
belongs to your account of what happens. jim beam doesn't
spoke-line-correct manually and doesn't think they bend much when you
tighten them either.

My own small experience is closer to yours on this matter, which is not
to say that I disbelieve jim beam.

[...]
>> 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.

Indeed.

 

[Ben C]

On 2007-09-10, jim beam <[email protected]> wrote:
[...]
> yes, there can be a small residual stress at the initiation point, but
> if we _observe_ fatigue initiating at a point where there is supposed to
> be /compressive/ residual stress, and we sometimes do, then clearly
> residual stress is a great big red herring.

Yes, and this is a good argument.

I have one more question, which I've already asked but am still not
completely sure of the answer to.

If the elbow _is_ bent again during the build (which many people do do,
and can just happen) what happens to those manufacturing residual
stresses?

I would have thought they would be little more than a ghost or an echo.
We've bent the wire again-- everything's moved, the outer skin has been
stretched and compressed all over again.

 

[Peter Cole]

jim beam wrote:
> 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.

Why don't you post your "explanation" then?


>
>
>>
>>>>>
>>>>>>
>>>>>> 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.

No kidding, but they're both defects, flaws, stress concentrators --
take your pick -- or perhaps you'd like to explain why an "inclusion" is
a special form of defect from a fatigue POV?


>
>>
>>>
>>>>
>>>>>
>>>>> 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.

Used by the Japanese for auto bodies in the 70's. US by 80's, little
man. How can this be "expensive" if it's used in massive quantities in cars?

 

[Peter Cole]

jim beam wrote:
> Ben C wrote:
>> 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
>
> yes, there can be a small residual stress at the initiation point, but
> if we _observe_ fatigue initiating at a point where there is supposed to
> be /compressive/ residual stress, and we sometimes do, then clearly
> residual stress is a great big red herring.


The "we" above is actually *you*. You are the one who claims to have
seen elbow breakage on the outside over inside by a 4:1 margin. You also
claim that your spokes loosen after a short ride on a newly built wheel.
Both are symptoms of the spoke line not being corrected -- something you
also admit to not doing. The causality should be obvious.

A thread where this was beaten to death:
http://tinyurl.com/2xvaq9

Also contains the bizarre sub-thread where you display a total lack of
understanding of vectors and wheel mechanics.

 

[Peter Cole]

jim beam wrote:

> quantifiably, right? you have numbers?

> that's not what i'm saying at all. what i /am/ saying is that if a
> spoke is interleaved, if it goes slack, the tension from its partner
> will cause considerably more bending excursion than if it had not been
> interleaved.

You have numbers?

My numbers say that 2mm spokes crossed 2cm from the ends will produce a
maximum skin stress from bending of about 30MPa. Of course the slack
spoke won't bend that much during the wheel cycle, in fact it will bend
hardly at all, since, because it is slack, there is no longer much if
any force at the crossing.

 

[Peter Cole]

jim beam wrote:
> Michael Press wrote:
>> 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.
>
> that can't be what we find in spokes though because if it's at or even
> near yield, fatigue life is minimal.

If you do the math, you'll see that the tensile stress in a nominally
tensioned spoke (1,000N) is at least 300MPa (for 2mm spoke). This is
slightly above the published yields for 302 & 304 stainless. This
corresponds to a spoke elongation of about 0.25%, which is consistent.

Published endurance limits for 302 & 304 are roughly at yield, so the
superposition of residual stress is in a critical region for fatigue
life impact.

 

[Ben C]

On 2007-09-10, Peter Cole <[email protected]> wrote:
> jim beam wrote:
>> Michael Press wrote:
>>> 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.
>>
>> that can't be what we find in spokes though because if it's at or even
>> near yield, fatigue life is minimal.
>
> If you do the math, you'll see that the tensile stress in a nominally
> tensioned spoke (1,000N) is at least 300MPa (for 2mm spoke).

Is this how you did the math:

radius of 2mm diameter spoke = 1./1000 metres
area of spoke = A = pi*r**2 = 3.1415926535897929e-06
force / area = S = 1000 / A = 318309886.18379068
S/1e6 = 318.3098861837907

so 318MPa of stress.

If so then that's the axial stress on the whole spoke, never mind
elbows, right?

> This is slightly above the published yields for 302 & 304 stainless.
> This corresponds to a spoke elongation of about 0.25%, which is
> consistent.

I thought that to yield a spoke axially you needed a huge force, much
bigger than spoke tension? But these numbers imply that normal spoke
tension is enough or close to enough.

Seems weird. I thought Jobst did some experiments where he broke spokes
just by loading them axially and they failed at forces of around 1000 or
2000 kgf, or anyway something big.

Perhaps what I'm underestimating is the size of the gap between yield
stress and ultimate tensile strength.

 

[M-gineering]

Ben C wrote:

>
> so 318MPa of stress.
>
> If so then that's the axial stress on the whole spoke, never mind
> elbows, right?
>
>> This is slightly above the published yields for 302 & 304 stainless.
>> This corresponds to a spoke elongation of about 0.25%, which is
>> consistent.
>
> I thought that to yield a spoke axially you needed a huge force, much
> bigger than spoke tension? But these numbers imply that normal spoke
> tension is enough or close to enough.
>

It is unusual to anneal spokes prior to wheelbuilding. The wire has been
coldworked and your low yieldstrenght for SS doesn't apply

--
/Marten

info(apestaartje)m-gineering(punt)nl

 

[Ben C]

On 2007-09-10, M-gineering <[email protected]> wrote:
> Ben C wrote:
>
>>
>> so 318MPa of stress.
>>
>> If so then that's the axial stress on the whole spoke, never mind
>> elbows, right?
>>
>>> This is slightly above the published yields for 302 & 304 stainless.
>>> This corresponds to a spoke elongation of about 0.25%, which is
>>> consistent.
>>
>> I thought that to yield a spoke axially you needed a huge force, much
>> bigger than spoke tension? But these numbers imply that normal spoke
>> tension is enough or close to enough.
>>
>
> It is unusual to anneal spokes prior to wheelbuilding. The wire has been
> coldworked and your low yieldstrenght for SS doesn't apply

So what is the rough figure for yield stress of the stuff they use for
spokes?

 

[Bill Sornson]

Peter Cole wrote:

> Used by the Japanese for auto bodies in the 70's. US by 80's, little
> man. How can this be "expensive" if it's used in massive quantities
> in cars?

TRIM!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

 

[M-gineering]

Ben C wrote:
> On 2007-09-10, M-gineering <[email protected]> wrote:
>> Ben C wrote:
>>
>>> so 318MPa of stress.
>>>
>>> If so then that's the axial stress on the whole spoke, never mind
>>> elbows, right?
>>>
>>>> This is slightly above the published yields for 302 & 304 stainless.
>>>> This corresponds to a spoke elongation of about 0.25%, which is
>>>> consistent.
>>> I thought that to yield a spoke axially you needed a huge force, much
>>> bigger than spoke tension? But these numbers imply that normal spoke
>>> tension is enough or close to enough.
>>>
>> It is unusual to anneal spokes prior to wheelbuilding. The wire has been
>> coldworked and your low yieldstrenght for SS doesn't apply
>
> So what is the rough figure for yield stress of the stuff they use for
> spokes?

tensile strenght in the middle section varies from 1000-1600 N/mm2,
yield probably 70-85% or so. The ends are not as deformed so will give a
lower figure

--
/Marten

info(apestaartje)m-gineering(punt)nl

 

[Peter Cole]

M-gineering wrote:
> Ben C wrote:
>
>>
>> so 318MPa of stress.
>>
>> If so then that's the axial stress on the whole spoke, never mind
>> elbows, right?
>>
>>> This is slightly above the published yields for 302 & 304 stainless.
>>> This corresponds to a spoke elongation of about 0.25%, which is
>>> consistent.
>>
>> I thought that to yield a spoke axially you needed a huge force, much
>> bigger than spoke tension? But these numbers imply that normal spoke
>> tension is enough or close to enough.
>>
>
> It is unusual to anneal spokes prior to wheelbuilding. The wire has been
> coldworked and your low yieldstrenght for SS doesn't apply
>

OK, fair enough. I see the range for 302/304 goes from 200-1,000MPa
(0.2% offset) yield, from annealed to full hard.

Jobst's curves show non-linearity at around 2kN for 1.8mm spokes, so
that puts measured yield somewhere around there (~800MPa). So, with a
working tension providing 400MPa, we need to double that to enter bulk
yield. Of course, the higher the yield, the higher the possible residual
stresses.

I assume that hardening moves the endurance limit up with the yield
(certainly no more than that), so that puts the nominal spoke (with
static load) within ~400MPa of the endurance limit. Since that's around
1/2 of yield, it seems residual stress is within range, whereas the
stress from bending at spoke crossings isn't.

 

[Peter Cole]

Ben C wrote:
> On 2007-09-10, Peter Cole <[email protected]> wrote:
>> jim beam wrote:
>>> Michael Press wrote:
>>>> 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.
>>> that can't be what we find in spokes though because if it's at or even
>>> near yield, fatigue life is minimal.
>> If you do the math, you'll see that the tensile stress in a nominally
>> tensioned spoke (1,000N) is at least 300MPa (for 2mm spoke).
>
> Is this how you did the math:
>
> radius of 2mm diameter spoke = 1./1000 metres
> area of spoke = A = pi*r**2 = 3.1415926535897929e-06
> force / area = S = 1000 / A = 318309886.18379068
> S/1e6 = 318.3098861837907
>
> so 318MPa of stress.
>
> If so then that's the axial stress on the whole spoke, never mind
> elbows, right?

Yes.

>
>> This is slightly above the published yields for 302 & 304 stainless.
>> This corresponds to a spoke elongation of about 0.25%, which is
>> consistent.
>
> I thought that to yield a spoke axially you needed a huge force, much
> bigger than spoke tension? But these numbers imply that normal spoke
> tension is enough or close to enough.
>
> Seems weird. I thought Jobst did some experiments where he broke spokes
> just by loading them axially and they failed at forces of around 1000 or
> 2000 kgf, or anyway something big.
>
> Perhaps what I'm underestimating is the size of the gap between yield
> stress and ultimate tensile strength.

See my other reply. I quoted a number for fully annealed. Matching to
Jobst's curves indicates the actual spoke material is considerably
hardened, my mistake.

Very roughly speaking, a tensioned spoke is 1kN, begins to plastically
deform at 2kN and snaps at 3kN. (per his graphs).

 

[Ben C]

On 2007-09-10, Peter Cole <[email protected]> wrote:
[...]
> I assume that hardening moves the endurance limit up with the yield
> (certainly no more than that), so that puts the nominal spoke (with
> static load) within ~400MPa of the endurance limit. Since that's around
> 1/2 of yield, it seems residual stress is within range,

Do you have a number for the sort of magnitudes one would expect
residual stress from forming to be?

> whereas the stress from bending at spoke crossings isn't.

Still not sure how you're working that one out. Bending is harder to
work out because you have to estimate moments which to do with any
accuracy would require taking into account the details of the geometry
around the hub hole which is complicated and I would expect to vary from
wheel to wheel.