AS-4 carbon fibers reinforced polyetheretherketone (PEEK) composite materials have been widely used in aerospace industry because of longer fatigue life, high specific stiffness and strength. The thesis is aimed to investigate the residual strength, residual stiffness and mechanical properties of thermoplastic AS-4/PEEK composite laminates subjected to tension-tension (T-T) cyclic loading at room temperature.
We adopt modified diaphragm forming method by controlling temperature, pressure, vacuum and time conditions according to the obtained beast curing process to form composite laminates of low crystallinity, transcrystallinity and good fiber / matrix interfaces. Two common type of laminates are used, such as cross-ply [0/90]4S and quasi-isotropic [0/+45/90/-45]2S. Static tension test is performed to measure the elastic modulus and ultimate strength. And T-T fatigue test is conducted with maximum stress of 60% and 80% ultimate strength to find the residual strength and stiffness. Then, through the observation of failure surfaces of composite laminates we understand the failure initiation and mechanism by Scanning Electron Microscope (SEM).
The results of experiment can be concluded as follows. The ultimate strength, elastic modulus and fatigue strength of cross-ply composite laminates are larger than those of quasi-isotropic. As centrally notched, the net area of the specimen is reduced, the ultimate strength and fatigue strength of composite materials are lower. The residual strength, adopted to describe the damage process, is monotonically decreasing with increasing of applied cycles. It is found that the residual strength of cross-ply laminates is larger than that of quasi-isotropic laminates. However, the residual stiffness has little change with increasing of applied cycles.