Study On The Effect Of Microstructure Of PAN-based Carbon Fiber On Macro-mechanical Properties

Composites have a wide range of applications in aerospace,sailing and diving,island construction and other high-tech areas of national defense.Carbon fiber is one of the most important reinforcing phases for advanced composites.Its properties determine the properties of composites.Therefore,improving the mechanical properties of carbon fiber has an important practical significance,and according to this,we determine the purpose of this paper.The effect of the microstructure of Polyacrylonitrile(PAN)-based carbon fibers on tensile modulus and strength was studied in this paper.The relationship between the microstructure and the tensile modulus is studied by establishing the micromechanical model of PAN-based carbon fiber.The relation between the microstructure and the tensile strength as well as that between the microstructure and the tensile modulus is studied by simulating the stretching of the basic structural units of PAN-based carbon fiber.The main contents of this paper are as follows:Firstly,the two-phase micromechanical model of PAN-based carbon fiber is employed to obtain the relationships between the microstructure of PAN-based carbon fiber and its tensile modulus applying the Eshelby equivalent inclusion theory and Mori-Tanaka method.Factors affecting the tensile modulus of PAN-based carbon fiber include the orientation degree of crystallites,the volume fraction of crystallites,and the aspect ratio of crystallites.The effect of three factors on the tensile modulus of PAN-based carbon fiber is discussed and compared using the two-phase micromechanical model of PAN-based carbon fiber.This two phase micromechanical model distinguishes the difference of effect of the same kind of factors on the tensile modulus of different kinds of PAN-based carbon fibers.Secondly,the PAN-based carbon fiber is considered to be a three phase composites that is composed of crystallites,microvoids and amorphous carbon on the basis of the results of X-ray diffraction(XRD),small-angle X-ray scattering(SAXS)and the theory of structure of PAN-based carbon fibers and a three phase micromechanical model is established.The predicted tensile modulus and the relationships between the microstructure of PAN-based carbon fibers and the tensile modulus are obtained by applying the Eshelby equivalent inclusion theory and Mori-Tanaka method.Factors affecting the tensile modulus include the orientation degree of crystallites,the volume fraction of crystallites,the aspect ratio of crystallites,the volume fraction of microvoids and the aspect ratio of microvoids.The effect of five factors on the tensile modulus of PAN-based carbon fiber is discussed and compared using the three-phase micromechanical model of PAN-based carbon fiber.This three-phase micromechanical model distinguishes the difference of effect of the same kind of factors on the tensile modulus of different kinds of PAN-based carbon fibers.Finally,six basic structural unit models of PAN-based carbon fiber are designed by applying molecular dynamics according to Reynolds-Sharp PAN-based carbon fiber structure model.Molecular dynamics is also employed to carry out the tensile failure simulation of the six basic structural unit models.It is re vealed that defects of the crystallites and amorphous carbon,crystallite-crystallite and crystallite-amorphous carbon interfaces are the occurrence of cracks.The effects of microstructures and the arrangement of crystallites,amorphous carbon and micro voids on mechanical properties are clarified.