Effects Of Elevated Temperature On The Structures And Tensile Properties Of Pan-based Carbon Fibers

In recent years, carbon fiber reinforced resin matrix composites (CFRP) withexcellent properties of the high strength, light weight and corrosion resistance is widelyused in the field of civil engineering, such as the reinforcement of existing bridgestructure and other components and reinforcement of the new structure. Compared withthe traditional structural materials (such as steel), the performance of CFRP willdeteriorate at elevated temperatures, and even burn. The fire resistance performance atelevated temperatures of CFRP is one of the important factors affecting the applicationin Civil Engineering.The fire resistance performance at elevated temperatures of CFRP depends on thecarbon fibers and resin matrix. The field studies not only the properties evolution ofhigh temperature of CFRP and its structure, but also concerns the performance changeafter experiencing fire or other high temperature extreme situations. The paper willfocus on studying the post elevated temperature conditions, namely the second kinds ofcondition, and aiming at the properties evolution of fiber. In the elevated temperaturefire, the carbon fiber of CFRP surface is in oxygen rich environment, while the carbonfiber inside CFRP may be in the anaerobic environment. Based on this analysis, thispaper would adopt the two kinds of environment (air and argon), two kinds of hightemperature treatment effect (time and temperature) to study the evolution rule betweenthe structure and tensile property of carbon fiber after elevated temperature treatment,and build the mutual relationship model among the tensile properties, structure andelevated conditions(400℃-700℃) of carbon fiber.The results showed that in air environment, the mass of carbon fiber decreasedrapidly with the temperature increasing, and the carbon fiber completely decomposedtill to about739℃, while in argon environment, elevated temperature treatment did notcause carbon fiber quality decreasing obviously. In air environment, elevatedtemperature treatment caused the diameter of carbon fiber decreasing, the increase ofsurface roughness, density and C-C skeleton content decreasing gradually, surfacedisordered, internal pore size and pore surface roughness increasing, the crystallite sizechanging small, and layer spacing of the graphite increasing. While in argonenvironment, the diameter of fiber kept almost unchanged, the surface roughness offiber decreased because of the surface small grooves merging into the big grooves, theC-C skeleton and other functional groups content remained basically unchanged, but thedisorder caused by the surface microcrystalline defects increased, the pore size, surfaceroughness and the graphite layer spacing increased.With the elevated temperature treatment temperature increasing, the tensilestrength degradation rule of carbon fiber in air environment was consistent with the argon’. At the same temperature, when the elevated temperature treatment timeincreased, the retention rate of tensile strength in argon environment was higher than theair environment’. In air environment, the elevated temperature treatment resulted in thecarbon fiber surface oxidation etching, the tensile modulus decreasing, while in argonenvironment, the modulus was almost unchanged.The structure of carbon fiber can be simplified ideally into two-layer structuremodel (skin-core structure model), and the thickness of the surface layer was0.818μm.From the model, the decomposition temperature of carbon fiber at elevated temperaturewas about550℃, and the decomposition time was approximately6h. At the same timeaccording to the degradation mechanism of tensile properties of carbon fiber at elevatedtemperature, the degradation models of tensile strength under two kinds of conditionswith the heat treatment temperature and time increasing were derived.