Effects of Stress Ratio and Stress History on the Threshold for Fatigue Crack Propagation

Abstract

In the present paper, authors have performed an experimental study on fatigue crack propagation at threshold level to clear the effects of stress ratio and stress history upon the threshold stress-intensity factor (K-threshold). Fatigue tests were carried out under load controlled three-point bending condition using side notch type specimens.The K-threshold values under the different stress ratio conditions are obtained by load decreasing test method which can eliminate the influences of loading history. The results show that the K-threshold range decreases with the increasing of stress ratio. An empirical equation which represents the relation between K-threshold range and stress ratio is proposed based on the comparison among present results and other researchers' data.TheK-threshold values after crack propagation at constant stress range are obtained by load increasing test method under various stress ratio conditions. It is found that theKthreshold value after stress history increases with the increasing of stress range at stress history process. And this tendency is well characterized by using maximum stress-intensity factor at stress history process and at threshold. A simplified equation for the prediction of K-threshold value after given stress history of constant stress range is derived from the present study. Estimated K-threshold values by proposed equation show fairly good agreements not only with present results but also with other researchers' data.