FATIGUE CRACK PROPAGATION UNDER SERVICE LOADING, THE THIRD REPORT: ESTIMATING METHOD FOR FATIGUE CRACK GROWTH UNDER NARROW BAND RANDOM LOADING

It is essential to be able to estimate the fatigue strength of structural members under random in-service loading on the basis of conventional fatigue test data. In a previous paper by the authors, fatigue crack propagation under narrow-band random loading was studied and the relationship between fatigue behaviors under random loading and constant amplitude loading was discussed. The consideration of crack closure was assessed as indispensable to the examination of fatigue behavior under random loading. This paper deals with additional fatigue tests that were conducted to obtain data under different stress conditions. Crack closure in the specimens was automatically recorded over an extended period of fatigue cycles, using a strain gage method aided by a microcomputer system. The significance of crack closure in understanding crack propagation under variable-amplitude loading is emphasized and a method for estimating fatigue crack behavior under random loading is proposed.