16Mn钢低温疲劳裂纹起始寿命的定量预测

Modeling of Fatigue Crack Initiation in 16Mn Steel below Room Temperature

本文研究了16Mn钢的低温疲劳裂纹起始,并力图提出可定量预测低碳低合金钢低温疲劳裂纹起始寿命的新方法。根据本文的研究结果,低温疲劳裂纹起始寿命可由拉伸性能进行估算,并且不需要任何低温疲劳试验和附加的经验修正。

Although the works of Toblcr[2] and Stephens[1] show that the fatigue crack initiation (FCI) from notches in low temperature environment is controlled by the parameter △K/ p^(1/2), no expression of the FCI life has yet been reported. In this paper, an attempt is made to offer a new approach to predict the FCI life below room temperature. By means of this approach, much money and time will be saved because the FCI life can be predicted from the tensile properties of the material without additional empirical modification and without any low temperature fatigue test. Our research indicates that the parameter △Kp^(1/2) should be replaced by the equivalent stress amplitude △σ_(eqv) (=[2/π(1-R)]^(1/2) △ K/p^(1/2)) so that the effect of the stress ratio (R) can be taken into account. Our research further indicates that the stress cycles to initiate a 0.25mm crack in notched specimens of a low alloy steel(16Mn) in the temperature range from 213K to 298K can be presented as a function of △σ_(eqv). The theoretical analysis shows that the FCI resistances in the low cycle fatigue region and high cycle fatigue are, respectively, controlled by the FCI resistance coefficient C and the FCI threshold (△σ_(eqv)) _(th). The values of Ccan be calculated from the tensile propoerties and △σ_(eqv) _(th) depends mainly upon the endurance limit which is approximately identical with the tensile proprtional limit of the steel. It is shown that the predicted FCI life curves are in good agreement with the test data determined at or below room temperature.As theoretically predicted, when the temperature is reduced, the FCI life of the steel will increase initially due to the increase of the strength but, if the temperature is sufficiently low,the FCI resistance at higher cycli stress level will decrease because of great decrease in the ductility.