Fatigue is a function of cyclic loading. This loading works the material and what is know as work hardening takes place.
Therefore the material becomes stronger. Hoever, in doing so it becomes less elastic and over a period of time can become brittle.
This is why fatigue and brittle fracture are often linked. Note however, that they are separate and distinct processes. A part may fail and be labelled as a fatigue failure, but the failure will often be the result of both brittle fracture and overload (or tearing) This is why I made reference to the type and length of crack in my earlier post.
Regards,
Shuttlebus