Investigation On Mechanical Properties Prediction Of Multi-scale Fiber Reinforced Composites

Fiber reinforced composites are widely applied in aerospace,transportation and other fields because of excellent characteristics such as high-strength lightweight and designability.However,the fiber/matrix interface of traditional composites is weak,and the stress transfer efficiency between fiber and matrix is low,which seriously limit the further improvement of composites’ properties.In recent years,scientists pay attention to nano-filler modified composites,especially the multi-scale fiber reinforced composites(MFRCs)which are deposited or grafted nanofillers onto the fiber surface.Experimental results show that the interface properties and related mechanical properties of MFRCs significantly improve,compared with traditional composites.However,the current research on MFRCs mostly focuses on experimental characterization,and there is a lack of relevant theoretical methods to predict the key mechanical properties of MFRCs accurately and efficiently.Under the background,this thesis focuses on MFRCs and establishes effective analytical prediction models of MFRCs’ stiffness,strength and damping based on the mechanics of composite materials,viscoelastic mechanics and composite damage/fracture mechanics.Then the influence mechanisms of multi-scale structural characteristics and component material properties on the overall mechanical properties of MFRCs are expounded.The specific contents of the thesis are following:(1)Based on Mori-Tanaka approach and Bridging model,a generalized prediction model for MFRCs’ effective stiffness,as well as a model for predicting the three-dimensional effective stiffness of MFRCs with waved CNTs(CNTs-MFRCs)are proposed by using two-step homogenization scheme.The nano-fillers’ shape effect,the CNTs waviness effect,and two-scale interphase effects including the CNTs nano interphase and the nano-filler reinforced interphase surrounding the-fibers on elastic moduli of MFRCs are revealed.(2)According to the strengthening mechanism of MFRCs’ nano-filler reinforced interphase and the fracture law of multiple fibers in composites,based on the local load sharing criterion,and considering the effects of nano-filler reinforced interphase shear strength and stress concentration factor,a longitudinal tensile strength prediction model of MFRCs is established.Subsequently,the influence of nano-filler content and geometric parameters on the longitudinal tensile strength of MFRCs are analyzed.(3)Considering the friction energy dissipation mechanism of CNT/matrix interface and the viscoelasticity of matrix,the dynamic modulus and loss factor of CNTs-MFRCs are predicted by energy method based on the shear lag model.Then the relationship between strain amplitude,component material content,nanofiber size and the damping of CNTs-MFRCs is explored.These research results are significant to develop the relevant theories of MFRCs and provide positive guidance for the optimal design and rational application of MFRCs.