Fatigue Strength of Metallic Materials under Varying Repeated Stresses

As is generally the case, a large number of machine members are subjected to repeated stresses of varying amplitude under the mean stress level which is varying continuously, and some parts are subjected to occasional overstresses (the stresses over the fatigue limit under constant stress amplitude). It is important in both academic and practical sense to estimate the fatigue strength, i.e. the fatigue life and the fatigue limit of metallic members under the stress condition like this.A number of workers have investigated this problem, but none have established a rational formula which predicts the fatigue life and the fatigue limit of metal under varying repeated stresses. For the accurate determination of the fatigue strength in these cases, the new criterion and formula have been established by employing the idea of the fatigue damage and the stress history.The functions of fatigue damage and stress history of metallic materials were established in this paper from the experimental investigations, considering the phenomena of work hardening caused by the repetition of the microscopic plastic strains of metals. The formulas which give the analytical S-N curves and the calculating methods to predict the fatigue lives and fatigue limits of metallic members under alternating stresses of varying amplitude, were obtained by employing the functions of fatigue damage.The fatigue test results under alternating stresses of varying amplitude carried out by Nishihara and Yamada, and those by Dolan and others, were analyzed by applying the calculating methods described.And it has been made clear that the analytical fatigue lives and fatigue limits under alternating stresses of varying amplitude had a good agreement with the test results. So it is concluded that the fatigue strength of metals subjected to varying repeated stresses like this can be determined by the analysis introduced in this paper.