一种考虑拉伸保持效应的多轴疲劳寿命模型

通过定义考虑拉伸保载效应的CFI因子(creep-fatigue interactionfactor)，将拉伸蠕变损伤和疲劳损伤进行非线性耦合. 根据断裂实验的观察，针对拉伸主导的裂纹萌生、扩展及破坏的多轴疲劳问题，给出了一个基于临界面方法的能量型高温多轴疲劳寿命预测模型. 所给出的模型可对不同温度、不同载荷特点、不同保载时间的多轴疲劳寿命进行预测，模型的材料参数不依赖于温度和载荷. 并且此方法可以很方便地推广到其它因素主导破坏的高温多轴疲劳寿命预测. 通过拟合高温合金Udimet720Li单轴带保持时间的低循环疲劳(low cycle fatigue, LCF)寿命试验数据，得到了材料常数. 结合黏塑性有限元分析方法，对高温双轴带保载循环载荷下Cruciform试件的寿命进行了预测，预测结果基本落在2倍分散带内，达到工程的要求，证明了该模型的有效性.

A new multiaxial fatigue model considering the influence of tensile dwell time

The influence of tensile dwell time on fatigue was used to define a CFI (Creep -Fatigue Interaction) factor, that characterizes the nonlinear interaction of creep damage and fatigue damage. A high temperature multiaxial fatigue model with energy-type parameters based on the critical plane approach was proposed. The model considers both the experimental observation of fatigue fracture and the tensile dominant effects of crack initiation, propagation and fracture under biaxial fatigue loading. The model has a "unified" ability of modeling the multiaxial fatigue life under different temperatures, various loading properties and dwell time. The material life constants in the model are temperature-independent and loading-independent, and the method may be generalized to other dominant effects of fatigue to build multiaxial fatigue models. The constants for Udimet720 Li, a superalloy used for turbine disc, were obtained by fitting the test data under uniaxial low cycle fatigue loading with dwell time. The model was applied to predict the fatigue life of cruciform under biaxial cyclic loading with dwell time at elevated temperatures based on the stress-strain results by a viscoplastic finite element analysis method. Errors are within a factor of 2, that is, within the engineering requirement margins. The validity and accuracy of the model were verified.