单向C_f/SiC复合材料的弯曲疲劳性能

BENDING FATIGUE PROPERTIES OF UD-C_f/SiC COMPOSITES

对单向Cf/SiC复合材料进行了三点弯曲疲劳性能测试 ,得到了复合材料的应力 -寿命曲线 (S -N曲线 ) ,并对其进行线性拟合 ,得到疲劳最大应力与复合材料疲劳寿命的关系 ;考察了疲劳过程中刚度下降和疲劳裂纹产生情况 .结果表明在疲劳过程中复合材料的弯曲模量有 3个变化阶段 :首先在疲劳加载初期 ,弯曲模量的下降速度及幅度都较大 ;其次在弯曲模量下降到原始弯曲模量的 85 % (13 3GPa)后 ,其变化方式没有明显的规律可循 ,有时甚至可能上升 ;最后复合材料发生疲劳断裂时 ,模量将发生突变 .显微结构分析表明 :基体横向裂纹群的产生是疲劳断裂的独有特征 .它的产生是由于基体SiC的断裂应变小于碳纤维的断裂应变 。

The fatigue properties of UD-Cf/SiC composites were tested using the three points bending method. The curves of the maximum fatigue stress versus fatigue life were obtained. And the curves were linearly fitted. The macroscopic fatigue behavior was characterized in terms of stiffness reduction and fatigue cracks. The results show that the change of the flexure modulus can be divided into three stages in the fatigue experiment. Initially, the flexure modulus degrades quickly. After the flexure modulus decreases to less than 5% of the initial modulus (133 GPa), the change of the flexure modulus is ruleless. When the specimen is fractured by fatigue, the flexure modulus is decreased abruptly. According to the SEM observation, the matrix cracks are found to be the primary characteristic of the fatigue fracture, which are caused firstly by the crazed SiC matrix and then by the resulting stress redistribution. The reason why the SiC matrix crazed first is that the SiC matrix' fracture strains are less than the carbon fibers' fracture strains.