Micro Mechanical Study on Fatigue Damage Mechanism of Composite Materials

为测量复合材料层合板在循环载荷作用下的损伤累积，研制了一套计算机辅助测试系统。应力-应变图中的刚度降低和能量假设作为循环次数的函数被监测。得到了玻璃纤维、碳纤维、芳纶及其混杂纤维增强塑料疲劳损伤刚度退化的主曲线。讨论了混杂效应和设计准则。对单向、交叉铺设和准各向同性碳氧基复合材料层合板进行了静力和疲劳试验，讨论了层合板的S-N图和刚度退化曲线，重点讨论了分层和基质裂纹的扩展行为。基于断裂力学方法建立了循环载荷作用下的分层扩展模型。用能量释放率测量脱层时的应力应变集中。基于能量释放率的模型较好地解释了弹性模数对疲劳强度的影响。为了准确测量疲劳分层扩展行为，在恒定裂纹剪切位移速率下进行了端部缺口弯曲(ENF)试验。得到了第二型能量释放率与分层生长速率之间的幂函数关系。

Computer aided testing system was developed for measuring damage accumulation of composite laminates under cyclic load. Stiffness reduction and energy assumption in the stress versus strain diagram are monitored as a function of cycle numbers. A master curve of stiffness degradation of fatigue damage is obtained for glass, carbon, aramid and their hybrid fibers reinforced plastic. Hybrid effect and design criteria were discussed.Static and fatigue tests were carried out for unidirectional, cross plied and quasi-isotropic carbonepoxy composite laminates, and their S-N diagrams and stiffness degradation curves were discussed with special consideration on propagation behavior of delamination and matrix cracks.A model of delamination growth model under cyclic load based on the fracture mechanics approach. Stress and strain concentration at delamination was measured by energy release rate. Effect of elastic moduli on fatigue strength was well explained by the model based on the energy release rate. In order to measure the accurate behavior of fatigue delamination growth, end notched flexur (ENF) test was carried out under constant crack shear displacement rate. A power raw relation between mode II energy release rate and delamination growth rate was obtained.