A Fractographic Study of Fatigue Crack Propagation under Variable Stresses with Special Reference to Crack Closure Phenomenon (Part 2)

Abstract

Effects of stress change on the fatigue crack propagation of aluminum alloys (7075-T6, 5052-0 and 2017-T4) were studied in detail by means of striation analysis of the fracture surfaces and the results were interpreted on the basis of the crack closure concept. In the same way as the case of 2017-T4 alloy reported previously, a single increase or decrease of the maximum stress caused an acceleration or a retardation, respectively, of crack propagation of the 7075-T5 and 5052-0 alloys and these could be interpreted fairly well on the basis of the effective stress range above the crack closure level. Effects of repeated change of stress conditions were investigated with 2017-T4 alloy and the results were essentially the same as the case of a single change of stress. Two opposing effects of stress change, the acceleration by the maximum stress increase and the retardation by the maximum stress decrease, made the total effect small, so that the cummulative cycle ratio was close to unity (between 1-2).