Abstract: It has been observed that, the evolution process of short fatigue cracks in some metallic materials presents collective damage characteristics The cumulation of the damage is produced by a number of short fatigue cracks The extent of damage is not dependent on a single crack, but on the whole response of total short cracks For this situation, we adopted the method of the balance of crack number density to describe such an evolutionary process The basic consideration of the model is that, at a certai...