Originally Posted by
fietsbob
looks like a couple nicks in the edge got the stress riser started.
It certainly had quite a few little dings... this was on my foul-weather bike so it got beat up more than my best bike. BUT...
Looking at the pattern of discoloration on the cross-section view, it appears the initial crack leading to failure was on the front surface of the crank - the part that curves outboard - between the branding marks; it started in the middle and spread outward toward the edges. This agrees with the fact that there are two parallel cracks to the failure also seen from the front view that don't go all the way to the edge. Couple that with the fact that the force that causes that kind of stress is when you're standing on the pedal at the bottom of the stroke, and it seems the nicks had minimal participation in the break.
Had I been better at inspections, I likely would have noticed the three parallel cracks before it broke and changed it out earlier. Luckily it didn't snap when I was out of the saddle sprinting... it just kind of felt like my cleat was coming loose and then it let go - with the pedal and half the crank still attached to my shoe.