Abstract: Based on the standpoint to take for the crack size a also to be a damage variable like the damage variable D, by means of the bidirectional combined coordinate system, three kinds of calculating methods with respect to the small crack growth rate and its life of elastic-plastic materials：the single- parameter method, the ratio method and the multiplication method are suggested. A lot of new calculating equations are given. Besides a new concept has also been provided on the compositive material constants A1, B1, A and A, which are functional relations with each typic material parameters: the fatigue strength coefficient , the fatigue strength exponent , the fatigue ductility coefficient , the fatigue ductility exponent , average stress, average strain and critical loading time, etc. In addition, as an example a part of car is calculated, and its calculating results make out some comparisons. The calculating methods have practical significance to avoid unnecessary fatigue tests, to stint times, manpower and capitals, and are convenient for engineering applications in a degree.

Key words: Damage, Multiplication, Ratio, Single parameter, Small crack