Yet another broken spoke

[Peter Cole]

Ben C wrote:
> 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?

No, not really. My little experiment confirmed residual stress and its
direction in bent spokes and formed elbows, but I couldn't really
estimate magnitude.

This source says 40-90% of yield in one case:
<http://cedb.asce.org/cgi/WWWdisplay.cgi?9000025>

This one shows typical profiles, measured with slitting, much the same
way I did it.
<http://www.lanl.gov/residual/bentbeam.shtml>

>
>> 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.

Simple beam deflection model.

 

[[email protected]]

On Mon, 10 Sep 2007 10:04:38 -0500, Ben C <[email protected]> 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?
>
>> 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.

Dear Ben,

Jobst's original tests showed that 2 mm 14 gauge spokes began to fail
at about 700 pounds, 1.8mm 15 gauge at around 600 pounds.

His later tests a few years later showed failures at the same tension,
but revealed much more ductile metal:

"In contrast to tests performed for the first edtion of this book,
these spokes withstood substantial elongation before failure. Some
butted spokes stretched more than six millimeters without breaking, at
which point the test was stopped."

--p. 152, "The Bicycle Wheel," 2nd edition

Cheers,

Carl Fogel

 

[Jambo]

"jim beam" <[email protected]> wrote in message
news:[email protected]...
> quantifiably, right? you have numbers?


Why, you able to do basic arithmetic?


> 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.

Shift the goalposts, quick, before anyone notices... oops, too late....

>> 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...

What are you going to do with it though?
>
> the process is called "sherardizing"

Wow, so if I define "inclusion", I too can make up stories from my youth,
bleat ******** and pretend to be a "former metallurgist"?

Wow.

> 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.

Stays polished if it keeps getting polished, moron! Man, this sometimes
gets tiresome.

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

Red ****.

 

[A Muzi]

> 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?

Bill Sornson wrote:
> TRIM!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Family newsgroup, buddy
(or do kids still call it that??)
--
Andrew Muzi
www.yellowjersey.org
Open every day since 1 April, 1971

 

[clare at snyder.on.ca]

On Mon, 10 Sep 2007 16:07:17 -0600, [email protected] wrote:

You wanted proof spoke wear in the "elbow" region could be an issue?
I said I'd look in my stash and try to find a few samples - I did this
afternoon and took a few pictures. Getting good focus with my digital
camera wasn't easy - but I got a few reasonable shots. See them at
www.on-the-net.ca/worn spokes.htm
or try this link:
http://www.on-the-net.ca/worn spokes.htm

I've seen lots worse, including a fair number of broken ones with
obvious signs of the same kind of wear.


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

 

[[email protected]]

On Mon, 10 Sep 2007 22:54:22 -0400, clare at snyder.on.ca wrote:

>On Mon, 10 Sep 2007 16:07:17 -0600, [email protected] wrote:
>
>You wanted proof spoke wear in the "elbow" region could be an issue?
>I said I'd look in my stash and try to find a few samples - I did this
>afternoon and took a few pictures. Getting good focus with my digital
>camera wasn't easy - but I got a few reasonable shots. See them at
>www.on-the-net.ca/worn spokes.htm
>or try this link:
>http://www.on-the-net.ca/worn spokes.htm
>
>I've seen lots worse, including a fair number of broken ones with
>obvious signs of the same kind of wear.

Dear Clare,

I think that you snipped whoever asked you about spoke wear in the
elbow region--it wasn't me.

The pictures baffle me. I can't think of the kind of profile in a hub
hole that would cause such damage. It looks more like gouging with a
chisel than an elbow wearing and eroding.

The other odd marks on the spokes in two pictures also puzzle me.

The second picture shows a tiny round dent and a curved gouge, both
where I can't think of anything in a hub causing the damage. The tiny
round dent might be from manufacturing, but the gouge seems random.
Here's the picture with the two marks circled:

http://i15.tinypic.com/65z9nar.jpg

The third picture shows two more odd dents or marks, equally
mystifying. Here's the picture with those two marks circled:

http://i5.tinypic.com/5y35nrd.jpg

I'm very curious to see what others with more experience say about
those marks.

Cheers,

Carl Fogel

 

[clare at snyder.on.ca]

On Mon, 10 Sep 2007 21:38:21 -0600, [email protected] wrote:


>
>The pictures baffle me. I can't think of the kind of profile in a hub
>hole that would cause such damage. It looks more like gouging with a
>chisel than an elbow wearing and eroding.
>
>The other odd marks on the spokes in two pictures also puzzle me.
>
>The second picture shows a tiny round dent and a curved gouge, both
>where I can't think of anything in a hub causing the damage. The tiny
>round dent might be from manufacturing, but the gouge seems random.
>Here's the picture with the two marks circled:
>
>http://i15.tinypic.com/65z9nar.jpg
>
>The third picture shows two more odd dents or marks, equally
>mystifying. Here's the picture with those two marks circled:
>
>http://i5.tinypic.com/5y35nrd.jpg
>
>I'm very curious to see what others with more experience say about
>those marks.
>
>Cheers,
>
>Carl Fogel

Some of the marks may have been caused while removing, but on the
first picture you will note the mark you circled was parallel to the
other wear line shown - I suspect it was the two edges of the hub
caused the wear lines. Same on the second picture.


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

 

[[email protected]]

Clare who? writes:

> You wanted proof spoke wear in the "elbow" region could be an issue?
> I said I'd look in my stash and try to find a few samples - I did
> this afternoon and took a few pictures. Getting good focus with my
> digital camera wasn't easy - but I got a few reasonable shots. See
> them at:

http://www.on-the-net.ca/worn spokes.htm

> I've seen lots worse, including a fair number of broken ones with
> obvious signs of the same kind of wear.

If I'm not mistaken this is from a steel flange hub. I think you'll
find that theses nicks are made when the wheel is first tensioned and
that it isn't from wear. Not being able to see the inside of the
depression, I'm not certain. In any event, we weren't discussing
inexpensive galvanized spokes in steel hubs. As I said they have a
whole set of their own problems.

Please clarify the circumstances.

Jobst Brandt

 

[A Muzi]

clare at snyder.on.ca wrote:
> On Mon, 10 Sep 2007 16:07:17 -0600, [email protected] wrote:
>
> You wanted proof spoke wear in the "elbow" region could be an issue?
> I said I'd look in my stash and try to find a few samples - I did this
> afternoon and took a few pictures. Getting good focus with my digital
> camera wasn't easy - but I got a few reasonable shots. See them at
> www.on-the-net.ca/worn spokes.htm
> or try this link:
> http://www.on-the-net.ca/worn spokes.htm
>
> I've seen lots worse, including a fair number of broken ones with
> obvious signs of the same kind of wear.

steel hub flange, yes?
--
Andrew Muzi
www.yellowjersey.org
Open every day since 1 April, 1971

 

[A Muzi]

>> [email protected] wrote:
-lost-

> clare at snyder.on.ca wrote:
>> You wanted proof spoke wear in the "elbow" region could be an issue?
>> I said I'd look in my stash and try to find a few samples - I did this
>> afternoon and took a few pictures. Getting good focus with my digital
>> camera wasn't easy - but I got a few reasonable shots. See them at
>> www.on-the-net.ca/worn spokes.htm
>> or try this link:
>> http://www.on-the-net.ca/worn spokes.htm
>> I've seen lots worse, including a fair number of broken ones with
>> obvious signs of the same kind of wear.

[email protected] wrote:
> I think that you snipped whoever asked you about spoke wear in the
> elbow region--it wasn't me.
>
> The pictures baffle me. I can't think of the kind of profile in a hub
> hole that would cause such damage. It looks more like gouging with a
> chisel than an elbow wearing and eroding.
>
> The other odd marks on the spokes in two pictures also puzzle me.
>
> The second picture shows a tiny round dent and a curved gouge, both
> where I can't think of anything in a hub causing the damage. The tiny
> round dent might be from manufacturing, but the gouge seems random.
> Here's the picture with the two marks circled:
>
> http://i15.tinypic.com/65z9nar.jpg
>
> The third picture shows two more odd dents or marks, equally
> mystifying. Here's the picture with those two marks circled:
>
> http://i5.tinypic.com/5y35nrd.jpg
>
> I'm very curious to see what others with more experience say about
> those marks.

Looks like steel hub flanges to me. Probably with low tension and a
heavy rim. The spokes are occasionally snug against the flange but
being less than what we'd call tensioned aren't 'seated' in a hub as
most wheels ridden by r.b.t. readers. Steel hubs are simply pierced and
often have no concession to a radiused edge. The spokes show marks from
both sides of the flange.
--
Andrew Muzi
www.yellowjersey.org
Open every day since 1 April, 1971

 

[jim beam]

Peter Cole wrote:
> Ben C wrote:
>> 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?
>
> No, not really. My little experiment confirmed residual stress and its
> direction in bent spokes and formed elbows, but I couldn't really
> estimate magnitude.
>
> This source says 40-90% of yield in one case:
> <http://cedb.asce.org/cgi/WWWdisplay.cgi?9000025>
>
> This one shows typical profiles, measured with slitting, much the same
> way I did it.
> <http://www.lanl.gov/residual/bentbeam.shtml>

but the reason they're interested in residual stress in the large
section materials they're looking at is not fatigue - it's because of
the elastic distortion that occurs on machining.

>
>>
>>> 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.
>
> Simple beam deflection model.

 

[jim beam]

M-gineering wrote:
> 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
>
correct.

 

[[email protected]]

On Mon, 10 Sep 2007 23:34:12 -0500, A Muzi <[email protected]>
wrote:

>>> [email protected] wrote:
>-lost-
>
>> clare at snyder.on.ca wrote:
>>> You wanted proof spoke wear in the "elbow" region could be an issue?
>>> I said I'd look in my stash and try to find a few samples - I did this
>>> afternoon and took a few pictures. Getting good focus with my digital
>>> camera wasn't easy - but I got a few reasonable shots. See them at
>>> www.on-the-net.ca/worn spokes.htm
>>> or try this link:
>>> http://www.on-the-net.ca/worn spokes.htm
>>> I've seen lots worse, including a fair number of broken ones with
>>> obvious signs of the same kind of wear.
>
>[email protected] wrote:
>> I think that you snipped whoever asked you about spoke wear in the
>> elbow region--it wasn't me.
>>
>> The pictures baffle me. I can't think of the kind of profile in a hub
>> hole that would cause such damage. It looks more like gouging with a
>> chisel than an elbow wearing and eroding.
>>
>> The other odd marks on the spokes in two pictures also puzzle me.
>>
>> The second picture shows a tiny round dent and a curved gouge, both
>> where I can't think of anything in a hub causing the damage. The tiny
>> round dent might be from manufacturing, but the gouge seems random.
>> Here's the picture with the two marks circled:
>>
>> http://i15.tinypic.com/65z9nar.jpg
>>
>> The third picture shows two more odd dents or marks, equally
>> mystifying. Here's the picture with those two marks circled:
>>
>> http://i5.tinypic.com/5y35nrd.jpg
>>
>> I'm very curious to see what others with more experience say about
>> those marks.
>
>Looks like steel hub flanges to me. Probably with low tension and a
>heavy rim. The spokes are occasionally snug against the flange but
>being less than what we'd call tensioned aren't 'seated' in a hub as
>most wheels ridden by r.b.t. readers. Steel hubs are simply pierced and
>often have no concession to a radiused edge. The spokes show marks from
>both sides of the flange.

Dear Andrew, Jobst, & Clare,

These diagrams from the 3rd edition of "The Bicycle Wheel" may show
the cause of the gouging:

http://i16.tinypic.com/542bp10.jpg

Under tension, the spoke cants in the thin, sharp-edged steel flange
described by Jobst and Andrew.

Cheers,

Carl Fogel

 

[jim beam]

Peter Cole wrote:
> 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

there is no endurance limit on that material - there is no "knee" in the
graph, there is no dislocation locking mechanism like there is in
materials that have one.


> 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,

only in mild steel. titanium, the other common endurance limit material
is only 40% or less. the fatigue "limit" defined for stainless that can
sustain a given number of cycles is roughly 30% of yield.

> it seems residual stress is within range, whereas the
> stress from bending at spoke crossings isn't.

but fatigue is not observed to initiate at the regions where there could
be high residual stress in spokes...

 

[jim beam]

clare at snyder.on.ca wrote:
> On Mon, 10 Sep 2007 16:07:17 -0600, [email protected] wrote:
>
> You wanted proof spoke wear in the "elbow" region could be an issue?
> I said I'd look in my stash and try to find a few samples - I did this
> afternoon and took a few pictures. Getting good focus with my digital
> camera wasn't easy - but I got a few reasonable shots. See them at
> www.on-the-net.ca/worn spokes.htm
> or try this link:
> http://www.on-the-net.ca/worn spokes.htm
>
> I've seen lots worse, including a fair number of broken ones with
> obvious signs of the same kind of wear.

while impressive, that's not wear, that's formation marking from a
particularly ugly bending mandrel. fantastic way to create a stress
riser though!

 

[jim beam]

Peter Cole wrote:
> 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?

you're evidencing an adjustment in equilibrium if there's residual
stress present. but it does not evidence residual stress at the surface!


>
>
>>
>>
>>>
>>>>>>
>>>>>>>
>>>>>>> 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?

sure - if surface defects are controlled and kept below a certain
feature size, fatigue can be substantially mitigated.

http://www.flickr.com/photos/38636024@N00/340348242/

[that's not an explanation, just an illustration.]

but in searching for an explanation of why even mirror polished
materials still initiate fatigue at the surface, it was found that
fatigue was initiating at inclusions where they interrupted the surface.
removing the inclusions removed these initiators and fatigue life was
found to be improved again.


>
>
>>
>>>
>>>>
>>>>>
>>>>>>
>>>>>> 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?

but wasn't! not for bulk sheet. maybe you're thinking of oxygen lancing?

http://en.wikipedia.org/wiki/Basic_oxygen_steelmaking

 

[jim beam]

Peter Cole wrote:
> 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.

again, i'm /NOT/ talking about the bend at the crossing - spokes don't
break there. i'm talking about the effect the interleaving has on the
/elbow/. it causes considerable deviation from the straight line, and
this /has/ to be accommodated by an increased bending moment at the elbow.

http://www.flickr.com/photos/38636024@N00/1313347532/

 

[Peter Cole]

jim beam wrote:
> Peter Cole wrote:
>> Ben C wrote:
>>> 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?
>>
>> No, not really. My little experiment confirmed residual stress and its
>> direction in bent spokes and formed elbows, but I couldn't really
>> estimate magnitude.
>>
>> This source says 40-90% of yield in one case:
>> <http://cedb.asce.org/cgi/WWWdisplay.cgi?9000025>
>>
>> This one shows typical profiles, measured with slitting, much the same
>> way I did it.
>> <http://www.lanl.gov/residual/bentbeam.shtml>
>
> but the reason they're interested in residual stress in the large
> section materials they're looking at is not fatigue - it's because of
> the elastic distortion that occurs on machining.

So what? What difference does it make why they're looking at residual
stress? These are just examples of its presence, typical profiles and
possible magnitudes. It also confirms the accuracy of slitting and
looking at strain.

 

[Peter Cole]

jim beam wrote:
> Peter Cole wrote:
>> 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
>
> there is no endurance limit on that material - there is no "knee" in the
> graph, there is no dislocation locking mechanism like there is in
> materials that have one.
>
>
>> 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,
>
> only in mild steel. titanium, the other common endurance limit material
> is only 40% or less. the fatigue "limit" defined for stainless that can
> sustain a given number of cycles is roughly 30% of yield.

We've been over this before. There is lots of evidence of SS having an
endurance limit, including MS lectures and manufacturers specs and
application design notes. I have cited this in the past and you have
dismissed it by saying that you didn't understand the mechanism
therefore it didn't exist. If that was a valid basis for exclusion we'd
have a small world.


>
>> it seems residual stress is within range, whereas the stress from
>> bending at spoke crossings isn't.
>
> but fatigue is not observed to initiate at the regions where there could
> be high residual stress in spokes...

There can be high residual stress at the surface, and there typically
is. Look at the *actual profile* I posted in the link upthread.

 

[Peter Cole]

jim beam wrote:
> Peter Cole wrote:
>> jim beam wrote:
>>> Peter Cole wrote:
>>>> jim beam wrote:
>>>>> Peter Cole wrote:

>>>>>> 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?
>
> you're evidencing an adjustment in equilibrium if there's residual
> stress present.

Of course you're "adjusting equilibrium", at least in the sense that the
spoke reaches a new equilibrium -- otherwise, according to Newton, it
would keep moving.

> but it does not evidence residual stress at the surface!

Upon re-reading I see I didn't really post enough information to allow
you to reach that conclusion.

The spoke movement didn't start until I was close to 90% through. I only
stopped when the remaining section became too thin to support (the
nearly vertical) weight.

If you examine a typical residual stress profile (cross section) like
the one I posted, and consider the predicted change in forces as a slit
is made, you'll understand that I saw exactly what I should have seen.
It's unambiguous.


>>>>> 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?
>
> sure - if surface defects are controlled and kept below a certain
> feature size, fatigue can be substantially mitigated.
>
> http://www.flickr.com/photos/38636024@N00/340348242/
>
> [that's not an explanation, just an illustration.]
>
> but in searching for an explanation of why even mirror polished
> materials still initiate fatigue at the surface, it was found that
> fatigue was initiating at inclusions where they interrupted the surface.
> removing the inclusions removed these initiators and fatigue life was
> found to be improved again.
>
>
>>
>>
>>>
>>>>
>>>>>
>>>>>>
>>>>>>>
>>>>>>> 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?
>
> but wasn't! not for bulk sheet. maybe you're thinking of oxygen lancing?
>
> http://en.wikipedia.org/wiki/Basic_oxygen_steelmaking