Study On Thermo-Viscoelastic Properties Of Three-Dimensionally Braided Composites

It is very important to study the thermo-viscoelastic properties of three-dimensionally resinmatrix braided composites, since they are widely used in the long term and variable-temperatureenvironment. In this dissertation, the thermo-viscoelastic properties of3D braided composites arestudied by theoretical derivation, numerical calculation and experimental verification.Firstly,3D braided composites are divided into three kinds of cells, and the braiding yarns aredivided into representative volume cells. Then, the multi-scale model of3D braided composites isestablished. A periodic boundary condition for the finite element analysis is applied to the multi-scalemodel.Secondly, based on the multi-scale model, the viscoelastic properties of yarns are obtained fromthe viscoelastic parameters of resin matrix and the elastic constants of fibers. Based upon theviscoelastic properties of yarns and resin matrix, the three-dimensional viscoelastic constitutiverelationship of interior cells is derived. The numerical results compare well with the experimental data.The influence of the two independent micro-structural parameters, braiding angles and fiber volumefractions, on the viscoelastic properties of3D braided composites is investigated in detail.Furthermore, the time-temperature equivalent relationship of3D braided composites is obtainedfrom the equivalent relaxation moduli at different temperature. Based upon the equivalent thermalexpansion coefficient assumption, the thermo-viscoelastic constitutive relationship is derived. Thethermo-viscoelastic behaviors of3D braided composites in different temperature states areinvestigated.Finally, the steady-state viscoelastic deformation response to alternating stress load is studied.The complex compliance of3D braided composites is derived from the creep compliance, and therelationship between the complex compliance and angular frequency is investigated. Based on thetime-temperature equivalent relationship and the equivalent thermal expansion coefficient assumptionof3D braided composites, the steady-state strain response of3D braided composites to alternatingstress load with thermal effects is derived. The dynamic response derived in the thesis for3D braidedcomposites with thermal effects agrees well with the finite element analysis. The effect of braidingangles and fiber volume fractions on the steady-state strain response of3D braided composites isdiscussed.