On Sep 30, 5:55*am, jim beam wrote:
On 09/29/2009 10:01 PM, wrote:

Any material can fail if there is a flaw in the design
or manufacture. *Steel is no panacea, but neither
is anything else.

cfrp is highly fatigue resistant - it would not fail in this manner.

iIt's probably because it was steel
that the fork was "rideable" for a while even though
cracked,

that ben, is utter garbage. *any material undergoing fatigue is
"rideable" until it fails.

Ah, Beamer. It's always good to have you disagree
with me. When you follow up a post of mine with a
message of agreement, I feel the need to recheck
my calculations.

CFRP is highly fatigue resistant - if it's strong enough
and doesn't have any construction flaws. On the other
hand, so is steel. Saying a CFRP fork wouldn't fail in
this manner is a bit meaningless because the fork
failed from fatigue initiated by a construction error and
worsened by a stress riser
(a brazing problem most likely, possibly worsened by
chroming as you argue, but most chromed forks
don't fail, so that brings us back to an initial assembly
error).

CFRP parts obviously don't fail from brazing
errors, but they certainly can fail from construction
errors and stress risers. My impression is that CF
bike parts tend to have a fairly large reserve strength
because they'll break quickly once they start breaking,
so manufacturers want to avoid the start of breakage
at all. (The exception might be superlight wheels, but
those tend to get banged up in use, and I'm excluding
crash damage from this discussion.)

Any part with a fatigue crack or flaw is rideable until it fails,
the question is how long it takes from the crack becoming
significant until full failure. For example, this fork was
cracked halfway around and still rideable (sort of) and
it sounds like that was going on for several days. An
aluminum part that was cracked halfway around probably
wouldn't last that long before failing completely. This
is conjecture on my part since I have never cracked
an aluminum fork like that, but it seems to me that
with aluminum frames and parts, they make the transition
from cracked to doomed very quickly. This is by no
means an argument against aluminum parts - you won't
catch me riding with steel cranks.


but whether that is good or bad depends
on your ideas about fault detection. *(That is,
a piece that cracks instantly is bad,

that's not fatigue, that's simply fracture.

I meant a piece that transitions from fatigue crack
to total failure quickly. You don't want that because
by the time you figure out something feels wrong,
you're already on the deck. Bad old titanium BB
spindles were probably in this category. Not
necessarily because titanium is evil, it's just not
good in that application.


but a piece that
lulls you into still riding it around even though it's
about to finally give way and drop you is also bad.)

this is what is so dangerous about fatigue and why so many millions of
dollars are spent on detection every year - pre-failure symptoms are not
obvious and need skill, awareness and usually testing gear to detect.
undetected failure can be tragic. *everyone should periodically inspect
their bike for this kind of stuff.

Well, yes, though in this case the rider did notice something
funny beforehand. It's always hard to know what's just
voodoo and what's a real problem, but when the bike starts
acting odd it's time to start taking pieces off and wiggling
things. If nothing falls apart in your hands, you can always
put it back together.

Ben