| Carbon fiber has excellent mechanical properties and thermal conductivity,is the most important reinforcement of advanced composites and has been widely used in many fields such as national defense and civil industry.Accurately characterizing the mechanical properties and thermal conductivity of carbon fiber is very important for the preparation and application of high performance carbon fiber.In this paper,high performance carbon fibers were used as research objects,through the monofilament tensile test and multifilament tensile test method to test its mechanical properties,using indirect test method and laser flash method on the monofilament and multifilament thermal conductivity test.The tensile strength of T700,M55J and XN-90 carbon fiber monofilaments was 4.7,4.41 and 3.52 GPa,which were close to those values reported by the manufacturers.The Weibull distribution theory was used to fit the strength of monofilament.The smaller the Weibull modulus m,the greater the discreteness;the pitch-based carbon fiber XN-90 has the smallest modulus m and the largest discreteness due to its large modulus and brittleness.It shows a nonlinear relationship between stress and strain of carbon fiber monofilament.The modulus of T700 increases first and then becomes flat as the strain increases,and the average value of the modulus is 227.52GPa,which is close to its reported value.The modulus of M55J and XN-90 increases first and then becomes flat,and finally decreases slightly with the increase of strain.The higher the modulus,the larger the error measured by the machine.The strength of T700 multifilament was 4.98GPa tesed using the epoxy E54 resin.The surface active functional groups of M55J and K13D2U were insufficient so it was necessary to add additives to improve their bonding properties with the resin and the improved strength was 3.95 GPa and 3.68 GPa according to the measurement.The choice of the strain range of the carbon fiber multifilament sample has a great influence on the modulus value.For T700 and M55J of medium high modulus fibers,the modulus obtained from the strain range of 0.1%?0.3%is the closest to its reported value.For K13D2U of ultra-high modulus fiber,the modulus obtained from the strain range of 0.1%to 0.2%is the closest to its reported value.For the ultra-high thermal conductivity carbon fibers as K1100,K13D2U and K13C2U,the monofilament thermal conductivity value calculated by the formula ?= 1261/p is the most consistent one to the reported value from the manufacturer;for high thermal conductivity carbon fiber XN-90,the value calculated by the formula ?=4400000/(100p+258)-295 is the most consistent one to the reported value from the manufacturer;for the general thermal fibers as P55 and P25,the value calculated by the formula ?=1261/p is the most consistent one to the reported value from the manufacturer.The relationship between the thennal conductivity and the resistivity is ?= 52740/(100p+22)-26 obtained from the measured resistivity and the thermal conductivity of the carbon fiber monofilament with the correlation coefficient R=0.9964.The thermal conductivity of the laboratory-made HNU carbon fiber is 949W/(m·K)according to the new formula.The thermal conductivity values of T700,M55J and HNU multifilament fibers measured by laser flashers were 10.16,93.47 and 719.99 W/(m K),respectively.For carbon fibers with high thermal conductivity as M55J and HNU,there are a lot of carbon fiber debris on both upper and lower surfaces due to the preparation method,and during the testing the heat would pass along the fiber debris on the surface,which caused low values.For the carbon fiber composites materials sample,since a low fiber volume fraction resulting in a quite large deviation in the thermal conductivity.Increasing the fiber volume fraction,the fiber is inhomogeneous distribution in the sample.So,it does not apply the mixing rule and the values measured by laser flashers are not credible. |
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