The Study On Fatigue Life Of Composite Laminates After Impact

With the rapid development of aeronautics and astronautics technology, advanced composite material have gained extensive use in modern aircraft structures due to its low specific gravity, high specific strength and modulus, resistance to elevated temperature and fatigue, which meets well the high demand of aircraft industry towards manufacture techniques that possess elevated performance, low cost as well as reduced weight. However, when subjected to low-speed impact during manufacture and service, invisible damage tend to appear in the interior of laminated composite plate, especially in the situation of alternating and cyclic loading, the rapid expansion of damage which generally leads to a great loss in load capacity and fatigue performance, even catastrophic failure could occur which gives rise to potential safety hazard. As a result, the study of composite laminate’s fatigue life after damage is of great practical engineering significance to the increase of utilization for comoposite laminates and to the reduction of aircraft maintenance costs.Two types of energy low-velocity impact test and compression test after impact were carried out on a typical layer T300/5405 Bismaleimide composite laminates in this paper, damage propagation law was obtained and the effect of impact on the static compression strength of composite laminates was discussed through the experimental study; the compression-compression fatigue test of composite laminates after respectively by two different energy impact was finished under 5 kinds of stress level, the relationship between the damage propagation and the fatigue life was analyzed. The research shows that the residual fatigue life less than 1/10 of the specimens’ overall life, when the damage width along the direction perpendicular to the fatigue load exceeds half the width of specimen; whether the surface VID or the internal BIVD, the damage have experienced steady propagation and rapid propagation two stages; compared with the 4.45J/mm impact energy, the fatigue stress threshold of laminates’ damage propagation after 6.67J/mm energy impact is lower. According to the damage detection results, the impact damage area of laminates is equivalent to the corresponding aperture open hole by the open hole equivalent method, chose the rule of exponential function fatigue model of laminates with no hole, use the average stress failure criterion of orthogonal anisotropic materials containing open hole, put forward the correction coefficient, establish the residual fatigue life model of composite laminates after low velocity impact of different energy under different stress level, the model based on the fatigue life curve of laminates after the 4.45J/mm energy impact, and the correctness of the model was verified by the fatigue test of laminates after 6.67J/mm energy. At the last, the composite laminates containing impact damage was studied by numerical simulation analysis, the fatigue life of finite element model was predicted by introducing the damage propagation in the test, at the same time, the damage propagation process and fatigue failure of laminates after impact were simulated, the results of finite element analysis are good in agreement with test results.