基于Cocks-Ashby模型的多轴蠕变设计准则的局限性及其修正

On the Limitation and Modification of Cocks-Ashby Model Based Multiaxial Creep Design Criteria

多轴应力状态下结构的蠕变变形分析是高温装备强度设计的理论基础。经典的Cocks-Ashby多轴蠕变孔洞长大模型及其简化模型存在着严苛的适用条件及局限性,仅适用于描述高温低应力环境下以晶间蠕变孔洞长大过程为材料蠕变损伤主要形式的蠕变变形行为。根据其简化解得到的用于单-多轴蠕变延性换算的多轴蠕变因子则存在着在低三轴应力度段给出的计算值较试验值偏小的问题。在基于WITCHMANN对该问题修正工作的基础上,通过合理的进一步优化,一个新的多轴蠕变因子被得出。将新修正因子应用在一个处于简化恒定应力场中的发动机热端叶片算例上,计算出恒载下叶片的等效多轴蠕变应变分布。有限元计算结果表明在叶片模型的叶根中部区域存在着较大的三轴应力度及较小的多轴蠕变因子,使得叶根中部原本应变不太明显的区域也产生了较大的等效多轴蠕变应变,导致叶片计算出的潜在失效面积较未经多轴蠕变因子修正前显著增加。

Structure analysis of creep deformation under multiaxial stress state is the theoretical basis of high temperature equipment design. The classic cavity growth theory（CGT） Cocks-Ashby multiaxial power-law creep model and its simplification have the limitation and strict applicable condition that is only suitable for modeling the creep behavior whose main damage mechanism is the grain boundary voids growth. The multiaxial creep factor, which serves as the conversion between uniaxial and multiaxial creep ductility, in the simplified model has the problem that the values in low triaxiality are lower than the experimental data. Based on WITCHTMANN＇s work that gives a simple modification of this problem, a new multiaxial creep factor is proposed to give a better modification. A blade model has been built in a simplified static stress field to calculate the equivalent multiaxial creep strain distribution. Results show that the mid zone of the blade root has a relatively large equivalent multiaxial creep deformation because of the relatively large triaxiality and relatively small multiaxial creep factor, leading to significant increase of the total potential failure area after correction by multiaxial creep factor.