Experimental Study On Mechanical Properties Of 3D Printing Fiber Reinforced Composites

Wit h the extensive applicat ion of fiber reinforced polymer composites in aerospace and automotive lightweight,the demand for green manufacturing,personalized customization and complex component processing is increasingly urgent.The application of Fused Deposit ion Modeling(FDM)in fiber reinforced composite materials can meet the above requirements and make up for the deficiency o f traditional manufacturing technology.However,the inherent characteristics of this process often lead to the decrease of mechanical properties of the printed parts,which is also the key problem of 3D printing composites.In recent years,the research on the mechanical properties of 3D printed fiber reinforced composites has been gradually carried out,but the basic mechanical properties and interface failure problems of printed parts have not been fully explained,and the research is single content and not systematic.Therefore,this paper systematically studies the printing of short fiber and continuous fiber composite materials by FDM process.A series of experiments were carried out from the aspects of process and equipment principle,mechanical properties of printed filaments,anisotropy and fiber content controllabilit y of printed parts,interlaminar fracture toughness and interfacial fracture characteristics under mixed-mode I+II loading,the main contents are as follows:(1)The mechanical properties of two kinds of printing matrix materials(Nylon,Onyx)and four kinds of continuous fiber reinforced materials(Carbon fiber,Glass fiber,High-temperature & high-strength glass fiber,Kevlar fiber)were studied experimentally.The fracture of the wire after failure was characterized by Scanning Electron Microscopy(SEM),and the defects in the raw material and the impregnation effect of the fiber and matrix were characterized.The r esult s show that,compared with the pure Nylon matrix,the stiffness of short fiber reinforced Nylon(ONYX)matrix is higher,but the strength is lower.Among several continuous fiber reinforced materials,the stiffness and strength of carbon fiber are the highest,and the toughness of Kevlar fiber is the best.(2)The effects of anisotropy and different continuous fiber content on mechanical properties and failure modes of the specimen were investigated experimentally.In addit ion,the printing qualit y o f the specimen was characterized by microscope.The results show that the laying direction of continuous fiber has a serious effect on the mechanical properties of the whole part.The tensile strength of the specimen in the fiber direction can reach 600 MPa,while the tensile strength perpendicular to the fiber direction is only less than 30 MPa.For the performance of the specimen in the fiber direction,the strength and modulus increase wit h the increase of the fiber content.However,the performance of the printed specimens perpendicular to the fiber direction is independent of the fiber content,and all types of specimens have the same strength as the pure matrix,which indicates that the transverse fiber does not improve the mechanical strength.The inter nal printing qualit y depends on the type of material.(3)ENF and DCB experiments were used to study the mode I and mode II fracture toughness of 3D printed continuous carbon fiber.The results show that the mode I fracture toughness of continuous carbon fiber is not low,and the fiber bridging phenomenon was observed during the experiment,which makes the extension value of fracture toughness higher.Combined wit h the characterization of the printing qualit y in the third chapter,it can be seen that there is no obvious interface boundary between the fiber layers,and there is no gap between the printing wires,which indicates that the adjacent interfaces are fused.The mode II fracture toughness is low,which indicates that the shear resistance of the interface is poor.(4)Designed mixed ? and ? interlaminar fracture pattern sample,and the interface material type and fiber direction were taken as experimental variables.The mechanical response and failure characteristics of different interfaces under mixed-mode I+II loading were studied.The results show that,the different material types of interfacial mechanics response,failure mode is complex and may occur in other than the middle layer.The stiffness of the specimens wit h a carbon fiber layer of0° in the middle is the highest,while that of the carbon fiber layer o f 90° is the lowest,because the carbon fiber layer of 90° cannot bear the load.The peeling stress at both ends of the contact surface is transferred to other layers throughout the thickness,leading to cracks that may first appear at other weak interlaye r interfaces.In addit ion,the cracks may transfer and merge in the process of propagation,so that the failure is transferred from one layer to another.