i was curious at what torque the bolt failed and 80 seems quite low for the bolt to sheer like that. how many torque cycles would you say this bolt had been under? i have to believe the fatigue was created by multiple cycles and/or a flawed bolt that finally let go.

 

I'm not an expert but I've experienced metal fatigue from a broken bicycle spoke and vibration-related motorcycle accessory mounts and have since done some research to understand and avoid future failures..

Apparently fatigue is not only related to "how many torque cycles", although that may be part of the explanation. Fatigue is caused by the load and flex caused by changes in the load. (there is probably a more accurate way to describe it). My bicycle spoke apparently broke not because of the torque when assembled, it was cause by the excessive flexing of the spoke because it was a little looser than the other 35 spokes, which took 1000's of miles to occur. i.e. The subject wheel bolt could have broken from the rotating and cornering forces over time and not from the assembling and disassembling of the bolts. Further, the new, higher 997 torque spec may be intended to prevent flexing of the tightened bolt, similar to my loose bicycle wheel spoke breakage.

Fatigue (material) - Wikipedia

The illustration, by coincidence shows a bicycle crank arm that broke, apparently unrelated (or not) to the pedal thread tightening. The slow, striated cracking effect, and the smooth-granular sudden failure.can be seen