| Stratosphere airship with a propulsion and control system is a new type of aerostat, and can float in the stratosphere for a long period of time. It has received more and more attention in the world due to its excellent military and civilian values. As is well known, the air density of the stratosphere is much lower than that of the troposphere(only 1/14), this indicates that larger size and lighter weight of the stratosphere airship is required to obtain enough lift. Therefore, the lightweight design is the most important criterion for the stratosphere airship structural design. In this respect some research work has been carried out abroad, however relevant information known to be available are very limited. The present work is concerned with the CFRP(Carbon Fiber Reinforced Plastic) composite skeletal system of the stratospheric airship. The model experiment, numerical simulation as well as optimization analysis were collaboratively carried out to investigate the stability of CFRP tube, the ultimate bearing capacity of CFRP truss joint, the bending and torsional properties of CFRP tube truss and the lightweight design of CFRP frame. The target is to establish corresponding refinement numerical simulation method and lightweight design method, and to provide technical support for the technical breakthrough of the stratosphere airship in China.The main content of this work can be summarized as follows:At the very beginning, the axial compression test of the short CFRP tube was conducted; corresponding ultimate bearing capacity and failure mode were also obtained. The finite element analysis method with the consideration of microscopic damage materials for analyzing the CFRP tube was established by the introduction of fiber bundle of transverse isotropic material model and the Hashin failure criterion; the accuracy of this method was proved through the axial compression test of short tube. Then, the axial compression stability tests of CFRP tubes with two different pipe diameters and five slenderness ratios were carried out. Through the application of arc-length method combined with Hashin failure criteria, the numerical simulation method for analyzing the axial compression stability of slender CFRP tube was established. The numerical simulation method was also verified by simulating the load-span deflection curves of test model and its failure mode. Finally, based on the proposed method, the parameter analysis on the axial compression stability of CFRP tubes with different slenderness ratios and the fiber volume contents was conducted. Thus, the formula of the stability bearing capacity of CFRP tube was obtained, as a result some recommendations for further research of this work and engineering applications can be offered.Taking CFRP joint as the research object, the microscopic mechanics numerical method was employed to study the modeling technology of 3D braided composites joint, including the selection of material constant, the determination of unit type, the division of the meshing grid, and etc. Consequently, methods for determining the ultimate bearing capacity of joint were discussed and verified by the comparison of simulated and experimental results. Then, based on Latin hyper cube sampling and multiple parameter sensitivity analysis method(Sobol’ variance decomposition), the parameter sensitivity analysis on three-dimensional CFRP composites joint was done, and the contribution rate of various parameters on the ultimate bearing capacity of the joint was quantitatively assessed. It was concluded that the pipe diameter and diameter-thick ratio had a significant influence on the sensitivity of the ultimate bearing capacity, while the effect of thickness ratio is relatively small. Finally, the optimized analysis on three-dimensional CFRP joint consisting of main parameters was carried out based on ANSYS software algorithm of zero order and first order algorithm. Taking the minimum volumes of joint as the optimization objective, the optimized joint under the same stress distribution condition had more uniform stress distribution, which indicates a good optimization result.Two CFRP truss models with an equilateral triangular section and a span of 3m was designed and constructed. The three point bending and cantilever torsion of static tests were then carried out. It should be noted that the three point bending condition was designed for a destructive test and the torsion condition was a stiffness test. The experimental results showed that the load-displacement curve of the truss under three point bending loads exhibits obvious bilinear characteristic, and the primary destructive behavior of the truss was the compression buckling of the upper chord. The multi-scale finite element method was introduced to analyze the bearding capacity of CFRP composite truss. It was found that under the three point bending condition, the error between the simulated and experimental results was within 5.3%, which proved that this model may accurately predict the buckling load and the deflection of destruction of the truss. Based on the above CFRP finite element model, the parameter analysis was conducted to investigate the effects of circular diameter of the truss, truss section number, thicknesses of chord and branch tube, and etc. The results showed that the inscribed circular diameter of the truss and the thickness of chord tube had a significant influence on the bending stiffness, while the number of the truss section mainly affect the bending ultimate bearing capacity of the truss. It is also found that the thickness of diagonal wall had less influence on the truss bending stiffness and ultimate bearing capacity of the truss.The stratosphere airship frame model with outer section of 220m×60m×60m was established, and the mechanical characteristics under two end supporting conditions were analyzed. A new classification criterion for the truss is established through the extraction of mechanical parameters(such as axial force and bending moment) of three control points located at the end and center of each single truss, and the whole airship can be divided into 7 typical truss frames. A new single-truss topology optimization method was proposed, which takes the "truss section" as the optimized variables and is combined with improved genetic algorithm. This optimization method does not necessarily lead to the bar as a unit compared with the traditional coding method, and thus the optimization calculation can have better convergence and stability; topology optimization analysis on 7 typical trusses has been done, and the optimized typical truss can be assembled into new airship frame. The results showed that the overall optimization effect can be more than 5%. Finally, a single-truss optimization program and its user interface have been developed using MATLAB software, for the convenience of the customer. |
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