| Summary: | In the present article, the degradation of the tensile properties of polyacrylonitrile (PAN)-based carbon fibers at elevated temperatures in air was studied experimentally and modeled. The tensile properties, mass loss, surface morphology, and elements and functional groups of carbon fibers were characterized. It can be concluded that the tensile strength and modulus of the carbon fibers decreased remarkably when the exposure temperature exceeded 500 °C. Oxidation at elevated temperatures etched the carbon layer from the skin to the core of the carbon fibers, leading to mass loss. According to the rule of mixtures, an exponential decay model was put forward to describe the degradation behavior of tensile modulus exposed to different temperatures and times. The thickness of the outer layer (Touter) of carbon fibers was obtained to be 0.818 μm. The ultimate exposure temperature was predicted to be 699.4 °C for 30 min, and the ultimate exposure time was 13.2 h at 500 °C. Furthermore, the time–temperature equivalence equation of tensile modulus was deduced. Through the introduction of the normalized oxidation degree, a degradation model of the tensile modulus at any exposure temperature (~800 °C) and time (~800 min) was also proposed. From the elastic mechanics theory for anisotropic solids, the degradation model of tensile strength exposed to elevated temperature was confirmed. It can be observed that the proposed model had good agreement with the experimental results.
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