As you said, Joe, that bolt has necked down - it is not a brittle failure, and it was tensile rather than bending. At a guess, it was either way too tight, or it had missed the heat treatment process and was a soft bolt instead of a high tensile one. It would be possible to find out which, by doing a hardness test on the bolt, but I think the latter is more likely from the look of it. Soft bolts neck down more than high tensile bolts, before failing in tension. Most likely the bolt was soft and was already necked down after being torqued up to the level that would have suited a high tensile bolt.
Another triumph for Chinese quality control? However, it is more than 4 years old - hopefully things have improved a lot since it was made.
