Research On Mechanical Properties And Engineering Application Of 3-D Braided Composites

The 3-D braiding procedure of composites is a new molding technology since 1980s'. In this method, the ancient braiding technology and the modern composites are combined organically to make the structure of composites unique. Using the 3-D braiding procedure in the manufacturing of composite, in the course of braiding, compositing and producing, materials are neither laminated nor machining, or small machining without the fiber damage. This can preserve the global properties of materials and overcome the shortage of laminae with low inter-layer strength and stiffness. the global strength and stiffness of composites are increased obviously, and the comprehensive characteristics of composites are also improved. At present, braided composites has been used in several high technologies, including national defense, astronavigation & aviation, industrial unit, medical instruments, automobile components, sport instruments. This paper goes deep into a thorough research on such subjects as braiding procedure, mechanical properties and high-tech engineering application for 3-D braided composites.The primary innovations of this dissertation are as follows:Based on the analyses of 3-D braiding procedure and the microstructure of perform, a type of fixed mesh method is proposed to simulate the procedure of 3-D braiding primarily. On the development platform which is composed of MATLAB, 3DMAX & AutoCAD VBA, the spatial mesh and the solid of the 3D-braided perform are plotted, and the full 3-D braiding procedure is simulated.Based on the analyses of the basic unit cell of 3-D braided composites, the three-cell model is adopted to predict the engineering elastic constants of the circular-section 3-D braided composites. According to the conclusion of mathematic derivation and the results of numerical calculation, a concept about quasi-transverse isotropic materials is declared primarily. The constitutive equations about this material are defined by six elastic constants. The characteristics of this material are between transverse isotropic material and orthogonal-anisotropic material. The circular-section 3-D braided composites can be regarded as the quasi-transverse isotropic materials. So this can make the mechanical analyses simple.A theoretic method is proposed to calculate the portrait draw strength of the 3D-Braided composite rectangle section beam based on the second-rank expression of Tsai-Wu rule. In this method, the 3D-Braided composites are regarded as traverse isotropy materials; thereby the tensor coefficients of every rank in Tsai-Wu rule can be decided by predigesting. So Tsai-Wu rule can be applied for the strength calculation of 3D-Braided composites.Based on the statistical model for the tensile statistical strength of unidirectional composite materials, a new method is proposed to calculate the tensile statistical strength of the 3-D braided composites. The numerical result shows that the tensile statistical strength of 3-D braided composites can be predicted according to this method.The probability distribution functions of residual stiffness and fatigue life for 3-D braided composites where derived primarily, and the determination method of some parameters in distribution functions mentioned above were given. Definite theoretical bases were presented to conduct fatigue life test and life forecast for 3-D braided composites.Through analysis of buckling and stress for a composite cylindrical shell, such technique targets as critical load, strength and stress distribution of 3D-braided composite cylindrical shells are calculated numerically at first. These results indicate that the effect of buckling to the 3D-braided composite CNGV is indubitable. Based on the 4-step 3-D circular braiding procedure and the theoretical analysis mentioned above, a mathematic model about the structural optimum design of 3D-braided CNGV is proposed. According to this model, the calculated example shows better effects.Through the analytics and the finite element method (FEM), the limited rotate speed and the natural frequency of 3-D braided composites high-speed flywheel are proposed theoretically and calculated numerically primarily. Because the 3-D braided composites has large specific strength and specific module, the flywheel can reach a high limited rotate speed enough to satisfy the need of flywheel battery by ensuring the strength of flywheel, and the natural frequency of flywheel is very large.