Li Longbiao
College of Civil Aviation, Nanjing University of Aeronautics and Astronautics, No. 29 Yudao St., Nanjing 210016, China.
Materials (Basel). 2020 Jul 24;13(15):3304. doi: 10.3390/ma13153304.
Fatigue limit stress is a key design parameter for the structure fatigue design of composite materials. In this paper, a micromechanical fatigue limit stress model of fiber-reinforced ceramic-matrix composites (CMCs) subjected to stochastic overloading stress is developed. The fatigue limit stress of different carbon fiber-reinforced silicon carbide (C/SiC) composites (i.e., unidirectional (UD), cross-ply (CP), 2D, 2.5D, and 3D C/SiC) is predicted based on the micromechanical fatigue damage models and fatigue failure criterion. Under cyclic fatigue loading, the fatigue damage and fracture under stochastic overloading stress at different applied cycle numbers are characterized using two parameters of fatigue life decreasing rate and broken fiber fraction. The relationships between the fatigue life decreasing rate, stochastic overloading stress level and corresponding occurrence applied cycle number, and broken fiber fraction are analyzed. Under the same stochastic overloading stress level, the fatigue life decreasing rate increases with the occurrence applied cycle of stochastic overloading, and thus, is the highest for the cross-ply C/SiC composite and lowest for the 2.5D C/SiC composite. Among the UD, 2D, and 3D C/SiC composites, at the initial stage of cyclic fatigue loading, under the same stochastic overloading stress, the fatigue life decreasing rate of the 3D C/SiC is the highest; however, with the increasing applied cycle number, the fatigue life decreasing rate of the UD C/SiC composite is the highest. The broken fiber fraction increases when stochastic overloading stress occurs, and the difference of the broken fiber fraction between the fatigue limit stress and stochastic overloading stress level increases with the occurrence applied cycle.
疲劳极限应力是复合材料结构疲劳设计的关键设计参数。本文建立了纤维增强陶瓷基复合材料（CMCs）在随机过载应力作用下的细观力学疲劳极限应力模型。基于细观力学疲劳损伤模型和疲劳失效准则，预测了不同碳纤维增强碳化硅（C/SiC）复合材料（即单向（UD）、正交铺层（CP）、二维、2.5D和三维C/SiC）的疲劳极限应力。在循环疲劳载荷作用下，利用疲劳寿命降低率和断纤维率两个参数表征了不同加载循环次数下随机过载应力作用下的疲劳损伤和断裂情况。分析了疲劳寿命降低率、随机过载应力水平及相应的出现加载循环次数与断纤维率之间的关系。在相同的随机过载应力水平下，疲劳寿命降低率随随机过载出现的加载循环次数增加而增大，因此，正交铺层C/SiC复合材料的疲劳寿命降低率最高，2.5D C/SiC复合材料的最低。在UD、二维和三维C/SiC复合材料中，在循环疲劳加载初期，在相同的随机过载应力下，三维C/SiC的疲劳寿命降低率最高；然而，随着加载循环次数的增加，UD C/SiC复合材料的疲劳寿命降低率最高。当出现随机过载应力时，断纤维率增加，疲劳极限应力与随机过载应力水平之间的断纤维率差值随出现的加载循环次数增加而增大。
