Study On The Relationship Between Macro-Microscopic Structure And Mechanical Properties Of Carbon Fibers

As one of the representative new materials,carbon fiber is widely used in aviation,aerospace,wind power generation,sporting goods and other fields because of its light weight,high strength,soft and easy to process,strong composite design and easy to shape.It is also one of the most important basic materials for weapons and equipment.In order to improve the quality of domestic carbon fibers and develop higher quality carbon fibers,a lot of performance tests and characterization have been carried out.How to improve the production process according to the performance characteristics of carbon fiber is of great significance to the development and research of carbon fiber.To this end,this paper selected 14 carbon fibers from 3 categories produced by 5 major carbon fiber manufacturers,in determining the reliability of the carbon fiber mechanical properties characterization methods,carry out the research from the relationship between microscopic structure and mechanical properties,hope to provide a certain amount of data base and the prediction model for carbon fiber product development,application engineering,and production.It is found that the cross-section of carbon fiber is irregular circle by using scanning electron fiber mirror.Current methods for testing and calculating the diameter of carbon fiber,such as the diameter measured by optical microscope is larger and the tensile strength is smaller,while the diameter measured by line-density method is smaller and the tensile strength is larger,which leads to some errors in the calculation of the tensile strength of carbon fibers.In this paper,a convenient method is proposed to obtain the cross-sectional area of carbon fiber by means of scanning electron microscope(SEM)images,which not only improves the accuracy of testing and analyzing the tensile strength of carbon fiber monofilaments,but also provides a reliable data basis for the subsequent analysis of this paper.The element content,graphitization degree and microcrystalline layer spacing of carbon fiber are measured and analyzed by element analyzer,Raman spectrum and X-ray diffraction.The correlation between carbon content,graphitization degree and microcrystalline layer spacing and tensile strength of carbon fiber is analyzed.In this paper,the one-way linear regression model is used for carbon content,and the multiple linear regression model is used for graphitization degree and microcrystalline layer spacing.The significance effect of the model is analyzed At present,there is a correlation between the carbon content of T300 grade carbon fiber and the tensile strength(significance level ? <0.1),graphitization degree andmicrocrystalline layer spacing no significant correlation with the tensile strength;there is a significant correlation between the carbon content of T700 grade carbon fiber and the tensile strength(? <0.05),a significant correlation microcrystalline layer spacing with the tensile strength,which conforms to the characteristics of the "skin core structure" model;there is a correlation between the carbon content of M40 grade carbon fiber and the tensile strength(? <0.1),graphitization degree and microcrystalline layer spacing both are related to the tensile strength There is a significant correlation between the tensile strength and the three-dimensional structural model of "onion skin" and "stump".Through X-ray diffraction and small angle X-ray scattering,microcrystalline width,microcrystalline thickness and micropore radius,fractal dimension of micropore of carbon fiber were measured and analyzed.Since microcrystalline and micropore are the common components of carbon fiber,the multiple linear regression model analysis of microcrystalline width,microcrystalline thickness,micropore radius,fractal dimension of microporeand tensile strength was carried out.The results show that microcrystalline thickness,micropore radius,fractal dimension of microporeof T300,T700 and M40 carbon fiber are significantly related to tensile strength.If carbon fiber is regarded as a composite structure with microcrystalline width as matrix,microcrystalline thickness as reinforcement phase and micropore radius,fractal dimension of micropore as defects,the conclusion of significant correlation with tensile strength is consistent with the meso-strength theory of composites.According to the Weibull distribution analysis of the tensile strength of carbon fiber,through the linear regression analysis and verification of micropore radius,fractal dimension of microporeand Weibull modulus,it is found that the correlation of T300 grade carbon fiber between fractal dimension of micropore and tensile strength is obvious,that between micropore radius,fractal dimension of micropore and tensile strength of T700 grade carbon fiber is significant,and that between micropore radius and tensile strength of M40 grade carbon fiberis significant.At the same time,it is verified that both the optical microscope method and the linear density-density calculation method will make Weibull calculation result higher and underestimate the dispersion degree of carbon fiber.Based on GRIFFITH's theory,an analysis and calculation method of fracture toughness of carbon fiber is given for the first time.Through the linear regression model analysis and verification of micropore radius,fractal dimension of microporeand fracture toughness,it is found that the method of testing fracture toughness given in this paper has some errors for T300 grade carbon fiber,and Themicropore radius,fractal dimension of microporeand fracture toughness of T700 grade and M40 grade carbon fibers were significantly correlated.In this paper,the correlation analysis between macro and micro structure and mechanical properties from the perspective of T300,T700 and M40 carbon fibers is given,obtained the significant structural parameters of three types of carbon fibers.The structure model and process mechanism of carbon fiber are verified,and the data prediction model is provided for the use of carbon fiber.