The Fatigue Strength under Varying Mean Stress

Abstract

The varying mean stress can occasionally be observed in service load acting on some structures or machine parts. It is important to estimate the effect of this varying mean stress on the fatigue strength of material. Hereunder is presented the report of experimental studies that were conducted according to a series of program of fatigue tests in which the mean stress changed periodically with long and short term. The results obtained are as follows.
The large stress range as the result of the mean stress change accelerates the fatigue damage to be caused by the repetition of small stress that succeeds it. The greater the change in the mean stress, the greater this effect becomes, and this influence gradually fades cycle by cycle. This phenomenon has been proved by measurement of the plastic strain at the notch root and of the fatigue crack growth rate. Though the mean stress changes in short term, the fatigue damage generated by the small stress cycles superposed is accelerated uniformly, but the transient aspect is not remarkable.
The methods to predict the fatigue lives according to the change in the mean stress are proposed. These are based on the fatigue damage effectively accelerated by the large stress range, and on the transient effect in the plastic strain behavior under the mean stress varying in amplitude.