A Study On Viscoelasticity Of Three-Dimensionally Braided Composites

Viscoelasticity is one of the most important properties of 3D braided composites. But there is few research in this subject. In this paper, the viscoelasticity of 3D braided composite has been studied.Firstly, the mesoscopic geometrical periodical unit cell structure of 3D braided composites is analyzed in details, using the 45°unit cell division method. The calculations of structural parameter determined by braiding process are come out. In this model, the cross section shape of yarn and the contact between them is fully considered, and the geometrical structure of unit cell is established.Secondly, the constitutive equation of epoxy is derived based on the theory of viscoelasticity. The theory of viscoelasticity of 3D braided composites is developed by the law of creep mixture. Then, the conductive equations, the matrix of creep compliance and the matrix of relaxation module of unit cell are established.Thirdly, the creep deformation of 3D braided composites is numerical simulated by finite element method, then the solution is analyzed by curve fitting method to get the creep compliance of composite; and then the relaxation modulus and the viscoelastic constitutive equation is obtained by the Laplace transformation.Finally, the effects of braiding angle and volume frictions to the viscoelastivity are analyzed in details through the numerical simulation for 3D braided composites of different braiding angles and different volume frictions. These analyses are done by the FEA software.The researches in this paper show that: the results of the conductive equations derived from theoretical calculation are similar to the results derived from the numerical simulation. The 3D braided composites with the smaller braiding angle and the higher volume friction has the better creep-resistance ability.