At a recent meeting a Blue driver had an axle fail because the fracture started out from a peg hole in the unused key position inboard of the third bearing. It was quite clear that the initial fracture had been around for a while ( showing signs of aging) and the final failure was caused by the crack extending from this.
Have a look at the accident reports from aircraft where the problem has been component failure. Quite frequently it is caused by something as small as a corrosion pit as small as a a human hair. This generates a stress concentrator, which creates a small fracture which gets bigger over time, possibly several outings.
Keep axles clean and free from scratches and rust.
It's also worth recognising that if you have had the grub screws shift around the axle, digging out a trackway, then not only is that a likely place for cracking to start but in order to generate the forces necessary to score out the axle, the axle bearings MUST have locked at some time. ( Normally you can hold the outer race with a fingertip while the inner race rotates at 2000 rpm.) The balls are likely to have got red hot in the process and shed metal into the bearing.
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