Abstract
The effect of stress change on the fatigue crack propagation of an aluminum alloy (2017-T4) was studied in detail by means of striation analysis of its fracture surface and the result was interpreted on the basis of the crack closure concept. The effect of stress change was large when the maximum stress was changed. The decrease or increase of stress caused the retardation or acceleration of fatigue crack propagation, respectively. The retardation of crack propagation after the maximum stress decrease was in good agreement with the analysis based on the crack closure. The acceleration of crack propagation accompanied with the maximum stress increase, on the other hand, could not be accounted for by the crack closure concept alone, but the effect of monotonic loading also had to be considered. The combination of these two effects gave reasonable agreement with the experiment. The effect of stress change under constant maximum stress was not as large as the effect of maximum stress change. A reasonable explanation was given also for this case. An analysis of striation morphology indicated that the crack closure appeared at least in the vicinity of the crack tip.
