Abstract: It is well known that fatigue is induced by the internal damage evolution of material. However the mechanism of damage evolution remains unclear yet. From the thermal disturbance of atomic motion, when the active energy of an atom is greater than a certain critical value, the atom will escape from its original equilibrium position and thereby a void will be generated. Under cyclic loadings, the voids generated by the thermal disturbance cannot be completely annihilated, so that the damage will be initiated or accumulated. From this view, the mechanism of damage evolution can be well explained, and the damage accumulation rate can be theoretically related to the rate of the statistical process of atomic escape. Therefore, this view provides the way to set up the damage evolution law with theoretical foundation. The evolution law based on this mechanism can well indicate the already known damage-life relationships of metals. Taking the experimental relationships between the damage and fatigue life under symmetrical cyclic loadings with different stress amplitudes into account, the damage evolution law is deduced in details. Furthermore, the accumulating rule of damage is also discussed on the basis of the proposed damage evolution law.

Key words: Damage evolution, Fatigue, Fatigue life, Mechanics of damage, Mechanism