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针对定向凝固(DS)镍基高温合金DZ125开展了850℃条件下不同缺口形式和不同理论应力集中系数(Kt)下的低循环疲劳(LCF)试验研究。利用弹塑性有限元分析缺口根部的应力应变场,并将传统临界距离理论(TCD)及其Kt修正形式引入SWT参数,以此开展缺口试件LCF寿命预测研究。结果表明:高温LCF强度同缺口几何形状关联不大,但具有强的Kt相关性;无论是将尖锐缺口试件作为校准试件还是对临界距离进行平均处理,传统TCD的点方法(PM)及线方法(LM)其寿命预测大于5倍分散带,且预测能力同缺口应力集中程度相关;改进TCD的点方法和线方法可得到小于2倍的分散带,且其预测精度与缺口几何形状无关。由于SWT参数可考虑平均应力(应力比)影响,故根据光滑试件和某Kt下缺口LCF试验数据便可以采用改进TCD预测其他缺口试件在不同应力比下的疲劳寿命,其应用简单、方便。
The low cycle fatigue (LCF) test under different notched forms and different theoretical stress concentration factor (Kt) at 850 ℃ has been carried out for DZ125, a directionally solidified (DS) nickel base superalloy. The stress-strain field of notched root was analyzed by using elasto-plastic finite element method, and the traditional critical distance theory (TCD) and its Kt modified form were introduced into SWT parameters to predict the LCF life of notched specimens. The results show that the strength of high temperature LCF is not related to the geometry of the notch, but it has a strong Kt correlation. Whether the sharp notch specimen is used as the calibration specimen or the critical distance is averaged, the traditional TCD point method (PM) and Line method (LM) is more than 5 times dispersion life prediction, and its predictive ability is related to the degree of notch stress concentration. The point method and line method for improving TCD can obtain less than 2 times dispersion band, and its prediction accuracy is not related to the notch geometry . As the SWT parameter can consider the influence of average stress (stress ratio), the fatigue life of other notch specimens under different stress ratios can be predicted by the improved TCD based on the data of the smooth specimen and the notch LCF under a certain Kt. The application is simple and convenient .