Fatigue failure

Fatigue fractures are generally considered the most serious type of fracture in machinery parts simply because fatigue fractures can and do occur in normal service, without excessive overloads, and under normal operating conditions. Fatigue fractures are serious because they are insidious; that is, they are frequently "sneaky" and can occur without warning that anything is amiss. Obviously, if service is abnormal as a result of excessive overloading, corrosive environments, or other conditions, the possibility of fatigue fracture is increased.
Let us consider the definition of fatigue that is commonly accepted:

The phenomenon leading to fracture under repeated or fluctuating stresses having a maximum value less than the tensile strength of the material. Fatigue fractures are progressive, beginning as minute cracks that grow under the action of the fluctuating stress.