Investigation Of Mechanical Properties Of 3D Braided Composites Based On Multi-scale Theory

Three-dimensional(3D)braided composite material is a high-tech material developed based on the 2D braiding technology,which has a unique structure.3D braided composite has the excellent advantages,such as high specific strength,high specific stiffness,ablative performance,integrity and design-versatility etc.It is widely concerned in the fields of high technology such as aeronautics,astronautics,defense,biomedicine and sports etc.With the growing maturity of the fiber braiding technology and the continuous expansion of the application fields,3D braided composite structures have gradually developed from the past secondary bearing structure to the main bearing structure,and become more and more important in various system structures in recent 30 years.The research on the mechanical properties of the 3D braided composites overall structure has become a research hotspot in the design and application of braided structures.However,due to the complexity of 3D braided composite microstructure,there is still much work to be done to study the mechanical behavior and damage evolution of 3D braided composites from the microscopic mechanism.Based on the microscopic configuration and a combination of mesoscopic and macroscopic,the stiffness and strength,thermal conduction properties,viscoelastic properties,and thermomechanical coupling behaviors for the overall structural of 3D fourdirectional braided composites are investigated by using numerical and experimental methods in this dissertation.Owing to the complex microstructure of 3D braided composites,the traditional finite element discretization will produce a large number of node degrees of freedom when the mechanical response of the braided composite overall structure is analyzed,which will lead to a huge computer memory requirement.In this paper,a new high effeciency multiscale calculation method is presented to predicte the overall strength of 3D braided composite structures.A macroscopic and mesoscopic finite element model of 3D four-directional braided composite is established.The stiffness,the macro and mesoscopic stress field distributions,and bending strength of 3D braided composites under three point bending are studied.In addition,the influences of the braiding geometric parameters on the effective elastic constants and the bending strength of 3D braided composites are also studied.The numerical results of the multi-scale method are validated by the experimental results.This method could also be extended to the 3D multidirectional braided composites even the heterogeneous materials with periodic structures and it provides a reference for the optimum design of composite materials.Based on the multiscale theory,the finite element equations for predicting the viscoelastic properties of 3D four-directional braided composites are derived.The macroscopic structural model,mesoscopic unit cell model and microscopic fiber bundle model are established.The equivalent relaxation modulus of the 3D four-directional braided composite in the Laplace domain is calculated at different scales.The equivalent relaxation modulus in the time domain is predicted by the least squares method for the 3D four-directional braided composite.The influences of braiding angle and relaxation time on the viscoelastic properties of the 3D four-directional braided composite are discussed.In addition,the time-dependent stress variations of 3D braided composites at macroscale,mesoscale and microscale are basically predicted by applying the special relaxation boundary conditions.Based on the multi-scale numerical calculation method,a steady-state thermal analysis model for the thermal conduction problem of 3D four-directional braided composites is established.The effective thermal conductivity coefficients of 3D four-directional braided composites are obtained.The influences of the braiding geometric parameters on the effective thermal conduction properties of 3D four-directional braided composites are also discussed.By applying two different types of boundary conditions(adiabatic boundary and non-adiabatic boundary),the thermal conduction behavior of 3D braided composites is further studied.The macro and mesoscopic temperature fields,heat flux distributions of 3D braided composites under different boundary conditions are predicted.Then the macro and mesoscopic thermal conduction mechanism of 3D braided composites are also analyzed.Based on the multiscale theory,a thermo-mechanical coupling analysis model for 3D braided composites is established.The stress distributions of the macroscopic and mesoscopic of 3D four-directional braided composite structure under thermo-mechanical coupling conditions are studied.The bending strength of 3D braided composites with different braid angles and different initial temperature changes are discussed,and the numerical results are verified by experiments.According to the thermoviscoelastic theory,a multiscale method for thermoviscoelastic properties analysis for 3D braided composites with initial temperature changes is derived.By establishing the thermoviscoelastic constitutive relationship of 3D braided composites,the equivalent thermal stress relaxation coefficients and equivalent time-dependent thermal expansion coefficients of 3D braided composites is predicted.The influces of braiding angle and relaxation time on the resulting thermoviscoelastic properties of 3D fourdirectional braided composites are analysed.With the thermo-mechanical coupling conditions,the equivalent thermal stress relaxation regularities of 3D braided composites at the macroscale,mesoscale and microscale are also studied.