The latest copy of Light Aviation has "a classic picture of a fatigue crack that has spread radially through a steel part, emanating from a stress concentration caused by a corrosion pit on the surface"
So any stress concentrator on a brake disc, whetehr it is a tooling mark, stone impact or corrosion pit (which can be smaller than a pin head) can be the cause of a fracture crack.
The second part of the caption goes on "Once the fatigue crack has grown to a critical size, the remainder of the cross section of the part has broken in one fell swoop, leading to a total failure", in other words if you can see the crack then it is time to junk the disc as you cannot predict if or when it will fail.
The previous page shows a Rotax 912 crankshaft that failed because it was twisted possibly through a propstrike, which might help explain why some kart axles snap, since the shock of hitting a kerb equates quite well to a propstrike. On a plane one only does it once, karts do it how many times a lap? Of course the crank doesn't have to fail totally there and then, the failure grows with time until the whole structure suddenly lets go.
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