The high cycle fatigue strength of crack-like discontinuities in metallic structures has been investigated using the critical distance approaches. Two methods have been employed, (a) the point method, and (b) the imaginary crack method. In the point method, the stress at a critical point within the material volume is chosen as the governing fatigue criterion. The effective parameter is the distance “d” from the material surface, which is a material property and the reference parameter is the fatigue limit. The imaginary crack method involves introduction of a sharp crack at the root of a notch and the length of the crack, “l0” assumed a material constant. The point method leads to a practical design rule that uses fatigue design curves expressed in terms of equivalent strain range versus number of cycles to failure. The equivalent strain is evaluated at a distance “d” from the crack tip. In the imaginary crack method, the effective crack length is taken as the sum of the actual crack and the material parameter “l0”. It is concluded that the high cycle fatigue has a volumetric character and the proposed methods introduce the volume effect in the determination of stress and strain fields as well as the fatigue life. Using the material parameter, the number of cycles to initiate a fatigue has been determined.
