Mesoscopic Researches On Elastic/Viscoelastic Properties Of Fiber-reinforced Composites With Graded Interphase

Fiber reinforced composites（FRCs）are a type of novel materials,which are of low specific weight,design versatility,simple molding process and superior environmental stability et al.From the early 20^th century to the present,FRCs have been widely applied in not only military engineering but also civil engineering.What’s more,research and development of advanced FRCs has become more and more urgent.FRCs are multiphase materials,whoes microstructure characteristics are prominent.As a result,its constitutive relationship is not only depends on the content of the component materials,but also the microstructure features.A number of experimental observation stated that there was an interphase layer between the fiber and the matrix in a FRC,the property of which varied gradly along the thickness,and it had great influence on macroscopic property of the FRC.Thus,in-depth investigation on effects of microstructure characteristics on overall properties of FRCs with graded interphase and searching for effective mesoscopic method for their properties prediction is challenging and of great significance for both theoretical research and engineering application.The dissertation covers two main parts:（1）in the region of linear-elasticity,a microscopic model for FRCs with graded interphase is established,and effective prediction schemes for the overall elastic property and the coefficient of thermal/moisture expansion（CTE/CME）are proposed,respectively.（2）Taken the interface imperfection into account,thermo-viscoelastic properties of FRCs with viscoelastic matrix,like fiber-reinforced polymer（FRPs）,are investigated.Additionally,the effect of the microscopic characteristics on the overall properties of FRPs are discussed,including creep compliance/relaxation modulus,storage modulus and loss factor.In the first part,firstly,based on the Eshelby principle and M-T theory（M-T theory）,using layerwise method,an extended of M-T theory is put forward.According to this novel mesoscopic theory,effective elastic properties of a FRC with graded interphase are predicted,meanwhile,influences of the content of constituents（fiber,matrix and interphase）,interphase gradient,aspect ratio of the fiber on elastic properties of FRC are discussed（See chapter 3）.Secondly,appling the extended of M-T method and according to linear elastic theory and superposition principle,a semi-analytical expression of CTE/CME is proposed,in which the influence of constituent content,aspect ratio of fiber,orientation of fiber,thickness and gradient of the interphase is considered（See chapter 4）.Thirdly,Based on Mesoscopic mechanics,a novel two-parameter agglomeration model is developed to take both fiber aggregation and porosity into account,in which the fiber weight fraction is used as an independent variable to express the volumetric composition.Meanwhile,an improved two-scale approach based on M-T homogenization theory is utilized to predict the effective properties of FRCs（See chapter 7）.In the second part of this dissertation,firstly,modeling weakened interface as linear elastic springs,microscript model for FRC with graded interphase and slightly weakened interface is established.Moreover,based on elastic-viscoelastic corresponding principle and time-temperature superposition principle,thermo-creep/relaxation properties of the FRP are obtained and their relation with the microstructural characteristics is thoroughly studied.In the analysis,the errors in previous analytical expressions of weakened tensors are corrected,and a efficient and accurate numerical method for calculating the weakened tensor is suggested,which is applicable for a wider range of inclusion’s aspect ratio（0（27）?（27）（10）?）（See chapter 5）.Secondly,based on thermo-viscoelastic mechanics,dynamic thermo-viscoelastic properties of FRPs with graded interphase and slightly weakened interfaces are studied.The effect of the filler content,grading index of interphase,the sliding compliance of interfaces,aspect ratio of fiber,loading frequency and environmental temperature change on their 3D effective storage modulus and loss factor are discussed（See chapter 6）.In this dissertation,a large number of research results have been published for the first time in the above-mentioned field,which helps to uncover the impacting mechanisms between FRCs’microstructure features and their effective elastic/viscoelastic properties.This research enriches and develops composites mechanics,interfacial mechanics,thermo-elastic mechanics and viscoelastic mechanics et al.,and has positive significance for theoretical research and practical application of advanced comosites.