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Really useful and interesting, thanks. Sounds like a genuine and serious problem with the design and/or manufacture of these cranks.
DrDave-
Sadly, I think the manufacturers themselves- at least their public facing departments- don't understand the causes of these failures either, and thus their blame of the operator is not generally dishonest (though still wrong!).
There used to be a rich collection of similar failures, with insightful commentary, here:
http://pardo.net/bike/pic/fail-012/000.html
It seems to have lost a few pages though. Still, an interesting read.
Edit- ah, here you go:
jodyonekenobi
Nearly all these pictures show fatigue failures. They had to have occurred over some time, and will have begun with a crack, invariably at a stress raiser - some sharp point in the crank, or a casting/forging error.
Looking at the broken surfaces, you can see where the face is dirty, is where the crack has been developing over weeks/months/years. The two halves fit back together neatly, also. The bright surface is a ductile failure, and shows how little material the component was still hanging on by. It’s surprising the story that these things tell.
The cause of these failures is virtually never rider error, or overload. If you tried to overload a new crank to destruction- by clamping it in a large vise and levering it with a breaker bar, say - it would bend, rather than crack. This persistent line from manufacturers that ‘you did something wrong’ is erroneous and unfair.