Hi Art,

Yeah, a hardness test would be an indicator of the strength of the bolt. I used to have a chart that told bolt strength by grade, this could easily be correlated to hardness for estimation purposes.

You generally cannot tell these things by visual looking, the bolt fracture surface though does suggest a fatigue crack initiated and grew until remaining section could no longer sustain the load. I'm leaning toward overtorque, but it's just an educated guess as this would shorten overall life in theory.

Good luck.

Mike