Study On Graft Modification And Tensile Failure Model Of Carbon Nanotubes In Flax Fiber Composite Materials

Flax fiber has advantages such as high specific strength,short growth cycle,environmental friendliness,wide sources,and low price.Compared with traditional reinforced fibers（such as carbon fiber,glass fiber,etc.）reinforced composite materials,flax fiber has weaker bonding performance with resin,and external loads cannot be effectively transmitted to the fibers through the resin,resulting in poor mechanical properties of flax fiber composite materials.Improving the mechanical properties of flax fiber composite materials through surface modification of plant fibers has become an important research direction for high-performance plant fiber composite materials.In addition,the single fiber model is a commonly used theoretical model for studying the tensile fracture properties of fiber composite materials.However,in the classic single fiber model,each fiber itself is brittle fracture,that is,the fiber immediately breaks after reaching strength.This paper constructs a progressive failure model（PFM）based on the bundle method,which introduces damage variables and factors affecting stress concentration coefficient,When the stress borne by the fiber unit reaches the strength of the fiber unit,the fiber does not directly fracture but is damaged first.The main research content and achievements of this article are as follows:（1）The method of grafting multi walled carbon nanotubes（MWCNT）onto the surface of flax fiber was proposed and studied,that is,under the action of ultrasound,multi walled carbon nanotubes were grafted onto the surface of flax fiber through silane coupling agent to form covalent bond.We studied and analyzed the effects of grafting treatment of multi walled carbon nanotubes on the mechanical properties of flax single fiber and flax fiber composite materials,as well as their interfacial bonding properties with the matrix.The surface morphology was observed by scanning electron microscope（SEM）and atomic force microscope（AFM）.And using photoelectron spectroscopy to analyze the elemental changes on the surface of flax single fibers before and after treatment.The results showed that the tensile strength of flax fiber,the tensile strength of flax fiber fibers before and after CNT grafting treatment.The results showed that the tensile strength of flax fiber,the tensile strength of flax fiber composites and the Shear strength of fiber matrix interface were improved after CNT grafting,but the modulus was not significantly increased.（2）A three-dimensional progressive failure model based on the bundle chain method was proposed and constructed,which can represent progressive damage in unidirectional composite materials and effectively predict the tensile properties of composite materials.Build a PFM model by first generating Representative Volume Element（RVE）,with each fiber unit in RVE having independent tensile strength σ_p,q^u.In this paper,Weibull statistical theory is used to assign different tensile strengths to each fiber unit.Then,the PFM model introduces damage variables D_p,q,and stress concentration coefficients SCF_p,q.After the fiber unit breaks,assuming that the stress transfer ability within the invalid length is damaged,the local effective stiffness is reduced by updating the damage variable.The stress concentration coefficient depends on two parameters,namely the radial distance between the two fibers δ_（q-j）and the plane position along the invalid lengthλ_（p-i）.Parameterδ_（q-j）Andλ_（p-i）can be parameterized through computational micromechanical studies or formulated through analytical methods.Finally,the constructed PFM model was used to predict the stress-strain curve of untreated flax fiber composite materials.The effectiveness and feasibility of the PFM model were verified by comparing it with experimental results.（3）The constructed PFM model was used to simulate the tensile failure of CNT grafted flax fiber composite material,carbon fiber flax hybrid composite material,and basalt fiber flax hybrid composite material.The calculated results of the PFM model were compared with the experimental results of this article,and the impact of size effect on the predicted results of the PFM model and its reasons were analyzed to further verify the feasibility of the PFM model.The results indicate that the PFM model can effectively predict the tensile stress and strain of linen fiber composites and hybrid composites treated with CNT grafting,and the RVE size effect has a significant impact on the predicted results.