Effects Of Microstructure Characteristics On Elastic Properties Of Plain-Weave Composites

This dissertation,in the context of the designs and analyses of composite structures of hypersonic vehicle,studies the effects of the microstructures of plain-weave composite on the elastic constants of the composite systematically and comprehensively.Firstly,the specimens of three typical plain-weave composites(E-Glass/Epoxy,C/Epoxy and C/SiC)to meso-and micro-scale microstructure imaging were manufactured by the different manufacture and machining processes.Meso-and micro-scale imaging experiments using Micro CT and SEM were then executed.The microstructures and those causes of the three composites in different scales are analyzed subsequently in detail by using the acquired images of Micro CT and SEM.After parameterizing yarns in textile composite,the feature parameters were measured on the slices of Micro CT images.A calculation model of shape parameters of yarn normal cross-sections based on ellipse projection method is proposed to solve the problem that the shape parameters data of yarn normal cross-sections cannot be acquired from slices directly.The period reference points of yarn are confirmed by the proposed real extreme slices searching method.A statistical analysis method of yarn feature parameters based on reference period method is developed to acquire the statistical properties of yarn feature parameters.Then,this method is utilized to analyze the statistical properties of yarn feature parameters of three typical plain-weave composites,including systematic trends(i.e.average values),global average values,standard deviations and correlation lengths.The difficulty of introducing the real variations in yarn path and shape into the stiffness matrix of yarn is solved by introducing variable metric coordinate transformation to the calculation model of elastic constants of yarn.The calculation model of the elastic constants of plain-weave composite considered real microstructures is established,which presents the bounds of effects of stochastic yarn feature parameters on the elastic constants of plain-weave composite.Three obvious advantages of the theoretical model are that a)the method does not require the application of anisotropic mechanics theory and FEM,but needs only the means and standard deviations of yarn feature parameters;b)the model can parametrically analyze the effects of the variations in the feature parameters of microstructures of textile composite on the elastic constants of the composite;c)the model can present analytical solutions in some special cases(such as single straight yarn).A method of measuring volume fractions of component materials of textile composite by using Micro CT experiments is developed in this research,which can determine the local fiber and yarn volume fractions of textile composite.The comparisons between the global fiber volume fractions of E-Glass/Epoxy plain-weave composite measured by Micro CT and ASTM D3171 Procedure G experiments illustrate the credibility and reasonability of the Micro CT experiment.The method measuring the volume fractions by Micro CT experiments can present global,local fiber and yarn volume fractions by Micro CT images in different scales,which can provide new solutions to measure the volume fractions of component materials of C/Epoxy and C/SiC plain-weave composites.By substituting the global average values of yarn feature parameters and the volume fractions of component materials into the established calculation model of the elastic constants of plain-weave composite,the effects of stochastic feature parameters on the elastic constants of E-Glass/Epoxy and C/Epoxy plain-weave composites are then studied.Parametric analysis is conducted for the effects of material parameters(including fabric architecture parameters,elastic properties and volume fractions of component materials)on the stochastic elastic characteristics of plain-weave composite.Linear discretized theoretical model on the basis of the ideal theoretical model of elastic constants of plain-weave composite is developed subsequently.By substituting the statistical properties of yarn feature parameters and the volume fractions of component materials of E-Glass/Epoxy and C/Epoxy plain-weave composites into the linear discretized theoretical model,the impact bounds of elastic constants of two composites affected by the variations of microstructures are then obtained.It can be concluded from the comparisons among the theoretical predictions,FE predictions and experimental measuments that a)the discretized theoretical model with taking account of real yarn stochastic features can predict more accurate elastic constants of the composite than deterministic models;b)the stochastic fluctuations of yarn feature parameters decrease the in-plane elastic moduli and increase the in-plane shear moduli and Poisson's ratios of the plain-weave composite;c)the stochastic elastic characteristics of plain-weave composite are closely related to not only the statistical properties of feature parameters,but also the fabric architecture parameters,elastic properties and volume fractions of component materials.The calculation model of the elastic constants of plain-weave composite can be extended to other type's textile composites.The results of this dissertation reveal the relation between the microstructures in meso-scale and the elastic constants of plain-weave composite,which are the foundations of researching the effects of the microstructures on the damage behavior and strength of plain-weave composite.