The Stress Analysis And Performance Optimization Of Composite Cylinder

As a special kind of container of gas, the composite cylinders are widely used in aerospace field, transportation industry, chemical engineering, medical industry and many other fields with its incomparable advantages. They have high specific strength and specific stiffness, good anti-fatigue performance, as well as a relatively small mass. However, the safety issue of the composite cylinders becomes increasingly prominent, and has attracted the attention of many scholars and the government. Under normal operation condition, the composite cylinders charge and discharge gas constantly, which results in a necessary for high strength and good anti-fatigue performance. Otherwise, due to the big difference of the mechanical properties of the two different materials forming the liner and the composite layers, the stress distribution of the composite cylinder is quite nonuniform. Therefore, in order to reduce or even eliminate the risk of dangerous accident, it is necessary to optimize the performance of the composite cylinder.Aiming to analyze the stress and optimize the performance of the composite cylinder, a lot of work has been done. The main contents of the research and conclusions are showed as follows:(1) Based on the software ANSYS, a proper finite element model was established to describe the composite cylinder. After different forces were applied to the model, the stress distributions of the composite cylinder under the auto-frettage pressure, zero pressure, hydrostatic test pressure, working pressure and bursting pressure were obtained. Then the strength of the composite cylinder was proofread according to the strength failure criterion of composite cylinder. The results revealed that:the stress on the liner’s inner surface was higher than that on the outer surface; the stress in the fiber layers were much less than its tensile strength; the strength in the liner and the fiber layers met the requirements under the working pressure.(2) After an introduction of the auto-frettage theory, the stress distribution of the composite cylinder before and after auto-frettage under different working conditions were compared, based on which the conclusions about the necessity of the auto-frettage treatment were abtained. Then the optimization of the auto-frettage pressure was conducted. The results showed that:the auto-frettage treatment could effectively reduce the stress in the_ composite cylinder liner, better the distribution of the stress and increase the utilization of the fiber; the optimized auto-frettage pressure of the composite cylinder was 37.9MPa. After the treatment under the auto-frettage pressure, the composite cylinder’s bearing capacity increased by 11.93% with a 150% rise of the utilization of the fiber layers.(3) The fiber hybrid method was applied to the optimization of the structure of the composite cylinder. On the basis of the proposition of equivalent thickness ratio, analysis about the influence of the hybrid ratio and the hybrid mode on the stress distribution was carried. The results revealed that:when the hybrid ratio reached 2:5, the overall performance was the best of all; the interlayer hybrid was better than sandwich hybrid on improving the bearing capacity; after structural optimization, the strength of cylinder liner increased by 14.9%, while the utilization of the fiber layers increased by 124% under working pressure; the volume of the fiber layers decreased by 30.3% with a drop of the mass by 36.3%. The bearing capacity increased with the optimization of the thickness of the fiber layers.(4) The influence of the auto-frettage and the fiber hybrid on the composite cylinder’s anti-fatigue performance was investigated. Analysis was carried upon the change of the stress amplitude, averaged stress, allowable cycle index due to the change of the auto-frettage pressure and the fiber hybrid ratio. The results showed that:the auto-frettage could effectively improve the anti-fatigue performance of the composite cylinder by decreasing the averaged stress; after hybrid of the carbon fibers into the fiber layers, the stress amplitude and the averaged stress of the liner of the cylinder decreased while there was an increase of the allowable cycle index and a decrease of the accumulated damage. Thus, the anti-fatigue performance and the service life of the composite cylinder was improved.