Abstract
As one of the investigations of fatigue strength of metals under service load, the fatigue tests have been carried out in the case where mean stresses fluctuated sinusoidally. In these cases, the fatigue age and endurance limits are reduced as compared with those in the case of constant mean stress, but they do not depend on the fluctuating speed, in spite of the difference in the rates of the speed from 5 to 420cpm.
We have been able to estimate the fatigue age and endurance limits in these cases by means of the criterion on the fatigue damage and the calculating method, which were proposed by the authors. These stress patterns are analyzed in the same way as in the case of varying stress amplitudes, and the fatigue strength in this case is possible of being estimated with the results of fatigue tests of constant mean stresses. It has been confirmed that both the results of the experiments and estimations exactly agree with each other.
If σ=H(n)is the equation of the cumulative frequency of repeated stresses, the formula of estimation of fatigue age Nt is as follows.
Nt=-n0/m∫n00H(n)dnloge{1-m∫n00H(n)dn/∫n00exp[A·H(n)+D]dn}
where the constants, A, D and m are obtained from the results of fatigue tests in the case of constant mean stress, the n0 is the cumulative number of stress cycles in one of the periods of the fluctuating mean stresses.
