Effects of Cyclic Fatigue on Fracture Behavior at Practical Loading rates in Carbon Composites

在碳纤维增强碳复合材料（C/C复合材料）中，弯曲模态和剪切模态的疲劳条件差别很大。在弯曲时，疲劳随应力比的增加而加速，而在剪切时，疲劳随应力比的减小而加速。在某些复合材料中，疲劳在水中或高湿度环境中加速。在碳纤维增强环氧树脂复合材料（CFRP）中，即使在高加载速率下，也可以通过改变硬化剂来控制其断裂行为。然而，在不同的cfrp中，不同的硬化剂所检测到的应力和/或应变差异不大。因此，这些行为的变化似乎是由局部变形引起的。利用原位高速摄影技术，对界面对应力波反射和损伤起始的影响进行了评价：界面在某些情况下充当了损伤源。在C/C复合材料中，疲劳对裂纹行为的影响是肯定的：疲劳试验前后裂纹路径发生了变化。在夏比加载速率下，改变基体树脂对断裂能的影响较小，但在所有加载速率下，界面黏附都对吸附有影响。上述实验结果表明，在离轴疲劳载荷下，界面结合控制是重要的，界面结合应根据加载速率进行控制。通过实验验证了所要控制的材料参数。

In Carbon fiber reinforced carbon composites (C/C composites), fatigue conditions for bending and shear modes are much different each other. In bending, fatigues are accelerated with increasing stress ratios, but in shear they are accelerated with decreasing the ratios. In some composites, fatigues are accelerated in water or in high humidity. In carbon fiber reinforced epoxy resin composites (CFRP), fracture behavior can be controlled by changing hardening agent even at high loading rates. However, the stress and/or strain detected by gauges are not much different among the different CFRPs with different hardening agents. Therefore, the changes in the behavior seem to be originated from localized deformation. The effects of interfaces on the reflection of stress waves and on the initiation of damages are evaluated by using in situ high-speed photographs : The interfaces act as a damage sources in certain cases. In C/C composites, fatigue surely affects the behavior of crack behaviors : crack-path changes before and after fatigue tests. The changes in fracture energy are small at the loading rates of Charpy tests by changing the matrix resins, but interface adhesion affects the absorption in all loading rates.The above experimental results indicate that controlling of interfacial bonding is important in an off-axial loading of fatigue, and that the bonding should be controlled depending on the loading rates. The material parameters to be controlled are demonstrated through the experiments.