Fracture surface looks a bit too battered to believe this is a one-time overload. You should examine the surface under a stereo binox scope using fiber optic lighting to look for evidence of fatigue initiation and propagation (or lack thereof). I cannot begin to count the number of times bolt fractures were presented to me as overload which became apparent as fatigue only after viewing under the scope.
is the nut supposed to be bearing on a flat surface? the damage looks to be different than would be inflicted by a lockwasher.
is the bolt securing 2 substantial metallic components together?
is a gasket involved?
Are the complaints of the fasteners loosening?
If other's comments regarding fatigue indications are correct, it looks to me like part of the problem may be that the bolt is not being installed to provide the preload it is capable of providing.
It might be as simple as adding some nice thick flat washers to the Bill of Materials and adding a torque spec ( boooo ! hisss! ) to the assembly sequence.
