I've said the same thing before several times, probably not as well.
This is a "cyclic" topic so here is a prior related thread, there are more:
Archive-URL: http://search.bikelist.org/
Don's point is that with steel, if the cyclic stress is low enough, there will be no failure no matter how many cycles the article is subject to. This is not so with aluminium.
To design this way for steel I vaguely remember you use a "Goodman Diagram".
Joe Bender-Zanoni Great Notch, NJ "More confidence in 75 year old BSA cranks than 25 year old Campagnolo"
> Donald Gillies wrote:
> >
> > Richard has a good point about aluminum crank failure. Aluminum has
> > no lower fatigue limit. When you have applied XYZ stress cycles of
> > force PDQ, and F(XYZ, PDQ) > max, the crank WILL fail. Steel has a
> > lower fatigue limit - if you can keep your stress cycles below this
> > limit, a 531 steel frameset will last forever.
> >
> > In other words, in 100 years we may still be riding our cambio corsa
> > bikes, but by that time all the nuovo record bikes will be TOAST.
> >
> > - Don Gillies
> > San Diego CA
>
>
> Well not exactly, as there is plenty of documentation of steel cranks
> failing at the pedal eye just as aluminum cranks do.
>
> Crank failure is very low on my list of things that can or will harm me.
>
> I've always maintained my equipment as if it was a light airplane and
> inspected for cracks whether it was something made from steel, aluminum
> or carbon.
>
> Chuck "life will kill ya" Schmidt
> South Pasadena, Southern California
>
> .
> _______________________________________________
> I'm pretty sure that point #2 in your message is wrong. Aluminum
\r?\n> has *no*
\r?\n> fatigue limit. That is, for any level of stress it is subjected to
\r?\n> (nomatter how small), there is a corresponding number of cycles at
\r?\n> that stress
\r?\n> level that will result in failure.
\r?\n>
\r?\n> Variable stress levels, like those that occur during typical
\r?\n> riding, mean
\r?\n> that failure happen more slowly than if every bump produced the
\r?\n> maximumstress, and more quickly than if every bump were as
\r?\n> innocuous as the most
\r?\n> minor one....but fatigue failure from stress cycling *will* eventually
\r?\n> happen.
\r?\n>
\r?\n> It may well be that modern metallurgy, design, fabrication, and
\r?\n> qualitycontrol have pushed the point of fatigue failure out to
\r?\n> where it isn't a
\r?\n> factor that need play into our thinking about the practical
\r?\n> lifespan of a
\r?\n> bicycle frame, but an aluminum frame will *not* last indefinitely.
\r?\n> [I'massuming that you meant the shade of meaning for this word
\r?\n> that implies
\r?\n> infinitely...strictly speaking, to say that a frame made of any
\r?\n> materialwill last indefinitely is pretty meaningless, because
\r?\n> you're saying it could
\r?\n> fail at any moment.]
\r?\n>
\r?\n> I'm pretty certain that age factors in to these considerations
\r?\n> mainly in
\r?\n> that certain types of exacerbating factors, like crack initiation
\r?\n> flaws, can
\r?\n> be caused by time-based environmental factors (in both steel and
\r?\n> aluminum,btw). Age may also factor in as a complication for
\r?\n> materials involved in
\r?\n> some joining techniques -- epoxy, for instance, could have a time-
\r?\n> relateddegradation (but, without UV exposure, it's *really* a slow
\r?\n> degradationcurve).
\r?\n>
\r?\n> Steel does have a fatigue limit. That means that there is a level
\r?\n> of stress
\r?\n> to which one can subject it *forever* with no failure. That
\r?\n> doesn't mean
\r?\n> that steel isn't subject to fatigue failures, but it does mean that a
\r?\n> correctly designed frame that's ridden in a way that never exceeded
\r?\n> thefatigue limit would never fail. On the other hand, ordinary
\r?\n> riding may
\r?\n> produce stresses that exceed the fatigue limit of the material, and
\r?\n> everyone of those incidents do chip away at the fatigue life of the
\r?\n> frame.
\r?\n> I've probably confused everybody who doesn't have a materials
\r?\n> science or
\r?\n> mechanical engineering background.
\r?\n>
\r?\n> Scott Minneman
\r?\n> San Francisco, CA
\r?\n>
\r?\n> -----Original Message-----
\r?\n> From: classicrendezvous-bounces@bikelist.org
\r?\n> [mailto:classicrendezvous-bounces@bikelist.org] On Behalf Of Harvey
\r?\n> M Sachs
\r?\n> Sent: Wednesday, March 29, 2006 5:11 PM
\r?\n> To: Classic Rendezvous; OROBOYZ; jerrymoos@sbcglobal.net
\r?\n> Subject: Aluminum fatigue, was Re: [CR]More on Alan frames
\r?\n>
\r?\n> I'm not convinced that either Dale or Jerry is correct. Dale said,
\r?\n> "Anyway,if you accept the fact that aluminum, in almost any
\r?\n> variation, when used in
\r?\n> bike frames will have a distinctly shorter lifespan as compared to
\r?\n> steel,"
\r?\n> And Jerry maintains that: "most members with an engineering/metallurgy
\r?\n> background will agree that fatigue failures in aluminum are related
\r?\n> to the
\r?\n> number of stress cycles a structure is subjected to, not simple age."
\r?\n>
\r?\n> I don't have time to re-read Wilson/Papadopoulos tonight, but my
\r?\n> memory is
\r?\n> that current understanding is that aluminum failure is much more
\r?\n> likely to
\r?\n> be related to relatively small numbers of much-beyond-design
\r?\n> strains than to
\r?\n> cumulative large numbers of small strain cycles causing fatigue.
\r?\n>
\r?\n> >From what I recall of the analysis in Wilson/Papadopoulos, it
\r?\n> would seem to
\r?\n> be fairer to say:
\r?\n>
\r?\n> 1) Any frame that is improperly designed or constructed with
\r?\n> respect to its
\r?\n> materials and joining methods is subject to failure. Wrong alloys,
\r?\n> wrongbrazing, wrong annealing,...
\r?\n>
\r?\n> 2) Conversely, properly designed and built frames of steel or aluminum
\r?\n> should last indefinitely.
\r?\n>
\r?\n> 3) The failure style of different materials can differ. Steel
\r?\n> tends to fail
\r?\n> more "gracefully," with cracks growing and material bending, while
\r?\n> aluminumoften snaps without obvious warning (campy cranks...). The
\r?\n> failure styles
\r?\n> may differ enough to influence decisions by prudent buyers.
\r?\n>
\r?\n> We may well see premature failures of some brands/models/designs of
\r?\n> aluminumframes. Engineering learns from failures, painful though
\r?\n> that may be. I
\r?\n> would be quite sure that there are some out there that skate too
\r?\n> close to
\r?\n> the edge, and are simply imprudent designs. There are some with
\r?\n> materialdefects, and some with joining defects, and they will be
\r?\n> concentrated on
\r?\n> specific brands and models. I don't know whic hones, but I'm
\r?\n> willing to be
\r?\n> they will be off-topic. :-)
\r?\n>
\r?\n> FWIW, I have had esthetic objections to the Alan design simce
\r?\n> seeing one for
\r?\n> the first time. Alan chose to sacrifice tube diameter for a
\r?\n> conventionalappearance. The relevant moduli of steel and aluminum
\r?\n> dictate that similar
\r?\n> behavior will require much larger diameter tubes if you want to
\r?\n> work in
\r?\n> Aluminum. Their commercial and racing success suggests that riders
\r?\n> aren'tvery demanding about this similarity in "stiffness" and other
\r?\n> properties,but it just doesn't look right to me.
\r?\n>
\r?\n> harvey "I knows good designs when I see it" sachs mcLean va
\r?\n>
\r?\n> +++++++++++++++++++++++++++
\r?\n> Jerry Moos wrote:
\r?\n>
\r?\n> Not to dredge up past discussions, many of which featured spirited
\r?\n> debates,but most members with an engineering/metallurgy background
\r?\n> will agree that
\r?\n> fatigue failures in aluminum are related to the number of stress
\r?\n> cycles a
\r?\n> structure is subjected to, not simple age. SO... an aluminum
\r?\n> frame that
\r?\n> has gathered dust in a garage for 20 years since purchase will be
\r?\n> essentially as good as new, while one that has been ridden 200
\r?\n> miles a week
\r?\n> for the same twenty years will probably not have much life left.
\r?\n> <snip>
\r?\n> Dale Brown wrote:
\r?\n>
\r?\n> We have covered this topic before but it may have been a few
\r?\n> years ago, so
\r?\n> I will likely repeat myself a bit here..
\r?\n>
\r?\n> "In the old days" I sold quit a few of these bikes..
\r?\n>
\r?\n> Most labeled as Guerciotti, but also under other brands including Alan
\r?\n> especially as cyclocross bikes...
\r?\n>
\r?\n> Anyway, if you accept the fact that aluminum, in almost any
\r?\n> variation, when
\r?\n> used in bike frames will have a distinctly shorter lifespan as
\r?\n> compared to
\r?\n> steel, these Alans, in balance, did amazingly well. Yes, after use
\r?\n> somecracked and a few have had the bonding fail, but many are still
\r?\n> out there
\r?\n> and I know of no catastrophic failures.
\r?\n>
\r?\n> Time has matched on and I think they are not a modern product, but
\r?\n> just wait
\r?\n> a few years and all these welded aluminum bikes will start cracking
\r?\n> too.Actually, I think I am seeing evidence in the bike shop that
\r?\n> this is already
\r?\n> happening.