Constitutive Model And Additive Manufacturing Of Carbon Fiber Reinforced High Performance Thermoplastic Composites

High performance thermoplastics,represented by polyetheretherketone(PEEK),polyamide(PA),and their carbon fiber reinforced composites have excellent mechanical properties,thermal resistance and corrosion resistance.In recent years,they have been widely used in automobile industry,mechanical equipment,electronic appliances and aerospace fields etc.Short fiber and continuous fiber are two main reinforcements of carbon fiber reinforced composites.Short carbon fiber reinforcement composites are easy to fabricate components with complex structure.The mechanical properties of short carbon fiber reinforced thermoplastic composites are not only affected by the ambient temperature,but also orientated by the flow field during the injection molding process,which result in the anisotropy of the mechanical properties of composites.This brings challenges to the design and development of short fiber reinforced composites.Traditional manufacturing processes of continuous carbon fiber reinforced thermoplastic composites(CFRTPCs)require costly mold and fail to fabricate complex construction with customized fiber alignment,which limits wide adoption of CFRTPCs in mainstream applications.Therefore,to develop high cost efficiency manufacturing processes for high-performance CFRTPCs parts is desired by industry.In order to solve the problems mentioned above,the main work done in this dissertation is as follows:(1)The uniaxial tensile responses of PEEK prior to necking are studied at strain rates ranging from 10-5 s-1 to 10-1 s-1 and temperatures ranging from-60℃ to 140℃.The tensile responses exhibit nonlinear characteristics and the dependence of temperature and strain rate.With the temperature increasing,the yield stress and yield strain of PEEK dramatically decrease while the modulus decreases slightly.Based on the uniaxial tensile results,a phenomenological nonlinear viscoelastic constitutive model is proposed to characterize the stress-strain of PEEK before yielding considering temperature and strain rate dependence.The results show that the proposed constitutive model can accurately predict nonlinear tensile stress-strain responses and yield stress of PEEK over a wide range of temperatures under small strain.(2)Tensile tests of SCF/PEEK composites were conducted in various mold flow directions at 23℃,80℃,140℃,and 200℃.Results show that tensile behavior of SCF/PEEK is highly influenced by the temperature and fiber orientation.Both tensile strength and elastic modulus decrease with the increasing of temperature,and fracture strain increases with the increasing of temperature.A shell-core-shell structure across the sample thickness was observed by scanning electron microscopy(SEM).The anisotropy of SCF/PEEK composites is fiber orientation dominated,and the temperature-dependence of SCF/PEEK is matrix-dominated.At elevated temperature,fracture surfaces become ductile,and fiber-matrix interface becomes weak.Constitutive model was established to predict the stress-strain curves over a wide range of temperatures.Overall,the constitutive model could predict the temperature dependent stressstrain relationships well.Based on the high-temperature mechanical properties of PEEK,we employed micromechanics homogenization method and fiber orientation analysis to predict the elastic modulus and tensile strength of SCF/PEEK composites with different fiber orientations at high temperatures.(3)We proposed a novel additive manufacturing(AM)technique to fabricate CFRTPCs using carbon fiber prepreg sheets based on modified laminated object manufacturing.CCF/PEEK and CCF/PA laminates with various fiber alignments were additively manufactured by laser-assisted laminated object manufacturing(LA-LOM)technique and ultrasonic vibration assisted laminated object manufacturing(UA-LOM)technique,respectively.The effects of processing parameters on flexural strength of the composites were studied.Hot press postprocessing was performed to further improve mechanical properties of the composites.The proposed method exhibited superior mechanical performances and low voids compared with other AM methods.Overall,the proposed AM methods of CFRTPCs show great potential applications in industries.Based on micromechanics and the classical laminate theory,elastic modulus of the manufactured laminates were calculated,and the calculated results are in good agreement with experimental values.