Abstract
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.
Highlights
Carbon fiber-reinforced polymer (CFRP) composites are attractive for load-bearing structures in many engineering fields due to their superior mechanical properties and potentially high durability [1,2].In recent years, the application of CFRP in different shapes for strengthening or reinforcing civil engineering structures has increased [3,4]
A sharp reduction of the tensile strength was observed with the further increase of the exposure temperatures to 700 ◦ C
The carbon fiber possessed a tensile strength of about 2.78 GPa at 550 ◦ C, with a reduction of 41.4% compared to the control fiber; at 700 ◦ C, the carbon fibers were completely oxidized into a white-colored residue, and no strength was left at all
Summary
Carbon fiber-reinforced polymer (CFRP) composites are attractive for load-bearing structures in many engineering fields due to their superior mechanical properties and potentially high durability [1,2]. Young’s modulus of the carbon fiber decreased by about 20% when the temperatures increased from 400 to 700 ◦ C, while the tensile strength reduced by nearly 40%. They concluded that the degradation of tensile properties was attributed to the skin–core heterogeneity of the microstructure for carbon fiber. When CFRPs were applied in engineering structures to strengthen or repair concrete components, the potential resistance to elevated temperature was larger owing to the fire protection coating This provided a reference for the selection of the exposure temperature and time of carbon fibers at elevated temperatures in this paper. The degradation of tensile properties for carbon fibers exposed to elevated temperature (~700 ◦ C) and time (~10 h) was studied and modeled. The agreement between theory and experiment was verified through the carbon fiber tensile results
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