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