Prediction And Identification Of Thermal-dependent Mechanical Properties Of Braided Composites

Compared with traditional laminated composites,braided composites have better overall performance because of their interlacing of internal fiber bundles.In the field of Aeronautics and Astronautics,the application of composite materials has become more and more extensive.Because of the different thermal expansion coefficient of matrix and fiber,thermal deformation and thermal stress inevitably occur inside the material under high temperature service environment,which causes the macroscopic properties to change,resulting in difficulties in the design of the thermal structure of the composite material.Accurately predicting and identifying the thermo-elastic mechanical parameters of the composite material is one of the keys to solve the problem.The main research work of this article is as follows:Firstly,based on the theory of thermos-elasticity,a method for predicting the thermos-elastic parameters of braided composites is proposed.Imposed appropriate periodic non-adiabatic temperature boundary conditions and displacement boundary conditions,the two-scale finite element model is used to predict the thermal-related parameters of braided composites.A 2.5-dimensional braided composite material is used as the object,and a two-dimensional finite element model of the fiber bundle and 2.5-dimensional braided structure is established,successively.According to the parameter prediction procedure,the fiber bundle unit cell is firstly taken to predict the thermoelastic relevant parameters,and then the prediction results of fiber bundles are brought into the 2.5-dimensional structure unit cell to predict the thermoelastic relevant parameters of 2.5-dimensional braided composites.Secondly,for the study of structural sensitivity in thermal environment,the finite element modeling and analysis of structural dynamics is carried out by means of thermos-elastic beams and plate elements under thermal environment.The element stiffness matrix K_T considering the influence of temperature and thermal stress stiffness matrix K_?are deduced.Then,the sensitivity are analyzed by the complex function method on the thermos-elastic beam and plate.In order to verify the correctness of finite element modeling of thermal structure dynamics and sensitivity analysis,thermal modal analysis and sensitivity calculation are performed with the two-end fixed beam and four-sided simple support plate.Finally,on the basis of verifying the accuracy of finite element dynamic modeling,the applicability of sensitivity analysis in thermal structure is further verified.Finally,based on the results of the equivalent prediction on the thermos-elastic parameters,an equivalent model of the braided composites is established,and this predicted value is used as the initial value of the parameter identification.The vibration model matching and correlation analysis are performed on the equivalent model and fine model of the braided structure,and the relative sensitivity of the two-dimensional orthotropic material parameters is analyzed.All material parameters are selected as the parameters to be identified,and the relative sensitivity of the complex variable function is used for optimization calculation.At last,based on the thermal model results of the fine model and the equivalent model,the residuals of the objective function are constructed to identify the thermos-elastic parameters of the braided composite.