小孔对DD6单晶热机械疲劳的影响

Influence of small hole on thermal mechanical fatigue in single crystal superalloy DD6

通过开展同相(IP)和反相(OP)循环下DD6单晶带孔试样与光滑试样的机械应变控制热机械疲劳(TMF)试验,研究孔边应力集中与温度-应变相位角对裂纹萌生寿命的影响。结果表明:带孔试样裂纹萌生于孔边最大主应力位置,其寿命比相同名义载荷的光滑试样低约一个数量级;带孔试样的OP寿命均短于IP,这与光滑试样寿命趋势一致。结合基于滑移系的黏塑性数值模拟,获取不同试验条件下的单晶应力应变分布特征及其演化规律,并构建单晶TMF损伤与宏细观参量的关联。在此基础上,建立一种能够综合考虑应力集中与相位角影响的单晶TMF寿命模型,对光滑试样与带孔试样IP、OP TMF寿命预测结果基本落在试验寿命的2倍分散带内。

Mechanical strain-controlled thermal mechanical fatigue(TMF)tests under inphase(IP)and out-of-phase(OP)cycles were performed on specimens with/without small hole made of nickel based single crystal superalloy DD6, and the impacts of stress concentration induced by small hole and phase shift between the mechanical strain and temperature on lifetime on crack initiation were studied. The results indicated that the life of the specimens with small hole whose cracks initiated at the location of maximum principal stress around the hole was one order of magnitude lower than that of smooth ones. Moreover,the OP life was lower than IP for specimens with small hole,which was consistent with that for specimens without small hole. The distribution and evolution of stress and strain in the specimens were obtained with the aid of numerical simulation with viscoplastic constitutive model based on slip systems. The correlation between the damage generated in single crystal superalloy under TMF loadings and the macro-/micro-parameters was identified. Based on the above-mentioned results, a new life model which could consider the effect of stress concentration and phase shift was proposed for the TMF life prediction. The predicted life of DD6 specimens with and without small hole under IP and OP TMF loadings was approximately within a factor 2 of the experimental life.