>Eventually,
>if used long enough at a stress level high enough, darn near everything will
>fail in fatigue even if it doesn't yield (bend).
At the risk of inciting all the real engineers on the list (I don't even play one on TV)....
There is something called a fatigue limit. The fiatigue limit is is the magnitude of stress that a part can be cycled through indefinitely and still not fail. There is some sort of asymptotic relationship between stress level and number of cycles to failure for any steel part. At a low enough stress, the steel will never break. This is not the case for aluminum. There is no fatgue limit for aluminum, so any amount of stress, applied enough times, will eventually cause failure. I have no idea what the metallurgical mechanism is that causes the two classes of materials to behave differently. I also don't know that anyone else does. This fatigue limit stuff might just pertain to "practical" engineering experiece and be empirically derived. It's engineering after all.
>P.S. If the longitudinal failure you're referring to is the one starting at
>the spindle taper hole, that looks like over-torquing (with grease perhaps)
>of the crankarm to me. Just a guess...
Yes, it was. Nasty thing that. I figure the tension in the taper hole had a role, and yours is certainly a good point.
Tom Dalton
Bethlehem, PA
In a message dated 4/30/02 3:05:11 PM Eastern Daylight Time, classicrendezvous-request@bikelist.org writes:
> Just had yet another look at Ric's images of failed arms. I saw one common
> denominator; all the arms looked thoroughly trashed, with lots of dents,
> scratches, abrasion, and corrosion. Except for the freaky longitudinal
> failure on the Campy arm (a real defect???) all these arms look like
> they've been pushed too far. Another interesting aspect is that many of
> the failure surfaces have black oxidiation half way across, or more. This
> suggests that the cracks existed for some time prior to the "catastrophic"
> failure. If the riders had their eyes open, these cracks could be spotted
> before a serious incident. Instead, the owners of these parts were
> pedaling around on cranks that were cracked half way through. Naturally,
> the last bit fails suddenly.
> Tom
> NortonMarg@aol.com wrote: In a message dated 4/27/02 12:57:36 PM Pacific
> Daylight Time, GPVB1@cs.com
> writes:
>
> << I think you may have just pointed out that any manufacturer's BB axle
> can
> break! If you ride long enough and far enough, you may end up with a total
> of
> zero manufacturer's axles that you will trust.
>
> IMHO, if you had an Engineer's inside view of the Automotive or other major
> Manufacturing business and saw the details of exactly what gets sold to the
> public, I fear you might never drive another car or use another toaster
> oven!
> Engineering and Manufacturing something for use by humans is always a
> compromise.
>
> Bottom line: any part/component/machine/thing can break. Some do fail; the
> vast majority don't. Most (yes, not all) that break do so after hundreds of
> thousands (or indeed millions) of cycles of use. Some break due to
> exceeding
> Design loads and/or stresses. Some finally fatigue due to old age at many
> times Design life (how long does anyone think that Tullio's Engineers
> intended a 1958 Record crankarm to last?). There are a zillion other
> reasons
> why something can fail. Ask Laurent Fignon if his infamous SR BB axle
> failure
> in the TdF was "catastrophic" or not. Ti (and many other "exotic"
> materials)
> fail that way. One of the best things about steel is that it rarely fails
> suddenly and catastrophically. Steel is real!
>
> You may want to see Damon Rinard's website for pics. of various
> manufacturers' broken bicycle parts
> (http//pardo.net/pardo/bike/pic/fail/FAIL-001.html for cranks and spindles,
> for example).
>
>
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