The Effect of Superimposed Stress of Higher Frequency on the Fatigue Strength of Aluminum Alloy

Abstract

This is the report of the study that was made of the behavior of fatigue strength of aluminum alloy 7075-T6 under a composite stress consisting of rotating bending stress and superimposed stress of higher frequency. The rotating bending stress is denoted by primary stress σ1 and the superimposed stress, by secondary stress σ2. The frequency ratio of the secondary stress to the primary stress is selected to about 12.Main conclusions are as follows. It was observed that the fatigue strength only depended on the frequency ratio of the primary and secondary stresses, but independent of the values of respective frequencies. The effect of the magnitude of secondary stress on the fatigue life was more or less depending on the magnitude of maximum stress σmax (=σ1+σ2). Let the σmax be higher than 1.5σw (σw; endurance limit), the fatigue life was observed to fall when σ2/σmax which is the ratio of the secondary stress to the maximum stress was more than 0.4, but not to fall when the σ2/σmax was less than 0.4. On the other hand, let the σmax be lower than 1.5σw, the fall rate of the fatigue life was slight in proportion to the decrease of maximum stress. In conclusion a convenient method of estimating the fatigue strength under composite stress is proposed.