Fatigue that the surfaces of the gear to show your photos to sift or fall into the cavity of the material or texture of the Cog is the contact and wear progression from small footprint to a big footprint quickly.
Fatigue on the surface of the tooth to show the form of disintegration. Loose or hollow materials. Or the surface of a tooth contact. And the wear is spread out over a larger footprint small footprint quickly.
Fatigue at the surface of the teeth to show the image of disintegration Or off the hollow of the material. Or the texture of gears is the face exposed. And wear will spread from a small to a large crack quickly.