Broken Dura Ace 7800 crank

jim beam wrote:
> Thomas Hood wrote:
>> I think this one hasn't been seen before.
>> Thomas Hood

> any chance of getting this:
> but in focus?

ok, so no dice.

but here's what you can see from the pic:
there does appear to be a small dark area of the fracture surface that
could be fatigue. however, the majority of the fracture is dull,
fibrous, and not perpendicular to the prevailing maximum strain. and of
course, it's bent, not cracked apart. these suggest significant
over-stress. my guess is that a crack started, probably due to surface
scratching, was spotted by the owner, and they decided to "accelerate"
failure by over-load.

under peak normally applied human loading, even if you took a saw and
cut through the crank 50%, you'd have a hard time bending the remaining
material [that's static load, not fatigue]. from what i can see, less
than 20% of that crank has fatigued, and while that can exceed K1c
depending on component and material, the fracture shown does not
evidence such brittle failure. conclusion, "aggravated overload".

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