Research On The Damping And Moisture Diffusion Behavior Of Flax/Glass Fiber Reinforced Hybrid Composite

Synthetic fiber reinforced composites have been widely used in the field of aerospace,ship and sport equipment.However,synthetic fibers,such as glass fiber and carbon fiber,have brought environmental problems when their service lives have finished.Natural fibers(flax,sisal and hemp)present low relative density,easy recyclability at life end,excellent acoustic performance,higher specific stiffness and strength.Therefore,they are potential alternatives to synthetic fibers in polymer composite materials in many industry fields in future.Despite of these advantages,the main drawback of nature fibers is the inherent susceptibility to the moisture absorption when they are exposed to humid environment.The moisture absorptions of hydrophilic cellulosic fibers degrade their mechanical properties,meanwhile introduce mismatch stress at the fiber/matrix interface.The latter could cause micro cracks in fiber/matrix interface or matrix and further degrades the mechanical performance of composites.To alleviate this unexpected moisture effect,nonhygroscopic fibers,e.g.glass or carbon fibers are suggested to be hybridized in nature fiber reinforced composite.This new hybrid composite possesses the balance of stronger mechanical properties,improved moisture resistance and reduced environmental impact.Beside,attributing their excellent damping property,the nature fiber composites have advantages when used in structures needing vibration and noise reductions.The nature/synthetic fiber hybrid composite have found growing applications in fields like car and construction.Currently,moisture diffusion behavior and damping properties of natural fiber/synthetic fiber hybrid composites still need to be investigated further.Well understanding these properties is the essential for wide application of this new sort composite.This thesis is aimed at investigations of the moisture diffusion behavior and damping properties of flax/glass fiber reinforced hybrid composites.The contents are as follows:(1)The moisture diffusion behavior of glass fiber reinforced unidirectional and 3D woven composites are studied.Firstly,the moisture diffusivity models of unidirectional composites are discussed.It is found that Kondo and Taki model which considers the effect of fiber random distribution is in good agreement with the experimental results in open literatures.Secondly,the moisture diffusion behaviors of 3D woven composites are solved by a multi-phases diffusion model.In the model,the unit cell of 3D woven composites which have complicated micro structures is divided into several parts.The moisture uptakes of these parts are solved by the finite difference method and the total moisture absorption of unit cell is a summation of the moisture uptake of each part.Finally,the experimental and finite element(FE)analysis are conducted to validate the theoretical model.(2)The longitudinal and transverse damping properties of unidirectional glass fiber composite are forecasted by Berthelot model,and the in-plane shear damping is obtained indirectly by the identification of the damping of 45~o unidirectional composite.Based upon these parameters,the stiffness matrix of the 3D woven composite is obtained through volume average approach.Damping properties of woven composites in dry state are predicted using complex stiffness method.Then the moisture concentrations for fiber tow and rich matrix region locating in various depths in 3D woven composite are predicted and thus the modulus and damping parameters are calculated.Free vibration tests of cantilever beam made of 3D woven composites are carried out to validate the proposed theoretical model.(3)Analytical moisture diffusion solutions for symmetric laminated materials with four and six plies are derived.Meanwhile a finite difference algorithm for analysing of more plies is also given.These models are proven validated through FE simulations.The effects of different moisture conditions used cross the interface of different phase layers are studied.The distributions of moisture concentration and normalized moisture concentration are obtained.The predicted moisture gains predictions for layered materials using these two conditions are compared.These proposed models are used in moisture diffusion analysis of flax/glass fibers hybrid composite with various ply patterns.The simulations are well agreed with the experimental data.(4)Ni-Adams and complex stiffness approaches are employed to forecast the damping properties of flax/glass fiber reinforced hybrid composites with different stacking sequences and hybrid ratios in both dry and wet conditions.The variation of moisture concentration along thickness on bending stiffness and damping are taken into account.The damping properties of hybrid composites in wet conditions are derived as a function of water absorption time.Both Ni-Adams and complex stiffness approaches have good agreement with experimental results.