Experimental and Modeling Study of the Evolution of Mechanical Properties of PAN-Based Carbon Fibers at Elevated Temperatures

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.

Structural Characterisation and Chemical Stability of Commercial Fibrous Carbons in Molten Lithium Salts

“…Another exception is the graphitised, ex-Rayon GR1, for which an increase in C content and a decrease in O content were seen, without clear net weight loss. These observations of changes in the elemental composition and weight losses evidence the sizing [49] or any other post-production modification of some CFs, and confirm the need to proceed to an additional de-sizing step before performing all characterisation analyses and chemical reactivity tests.…”

“…Here, only surface etching and smoothening were evidenced, which clearly indicates that LiOH alters the fibre morphology differently due to differences in chemical reactivity. As presented elsewhere [49,56], it was also observed that subsequent carbonisation and graphitisation treatments had an influence on the structural improvement of ex-PAN carbon fibres, which controlled morphological changes during air oxidation and chemical activation in KOH or NaOH. Here, a clear effect of the chemical reactivity in molten LiOH was only observed at higher temperature, above 650 • C. Thus, the studied carbonised ex-PAN CFs (CP5a750 and CP7a750) experienced surface smoothening and a shrinkage of the carbon fibre diameter of 2 µm on average (see Figure 12).…”

Mechanical property evolution and life prediction of carbon fiber and pultruded carbon fiber reinforced polymer plate exposed to elevated temperatures

“…On the one hand, carbon fibers with the length of 10 cm were prepared and the exposure condition was from 25 C to 700 C for 30 minutes in a muffle furnace in air and argon environments at a heating rate of 10 C/min as shown in previous report. 16,17,28 Meanwhile, the exposure at 300 C for 10 hours was also conducted in air to analyze the evolution of carbon fiber in CFRP plate. On the other hand, CFRP plates were exposed in a muffle furnace from 25 C to 300 C for 10 minutes in air.…”

“…The microstructure analysis of carbon fiber after the exposure included the surface morphology analysis, elements and functional group analysis, Raman spectroscopy, small angle X-ray scattering and Wide-angle X-ray diffraction, as shown in previous report. 16,17 3 | RESULTS AND DISCUSSION Figure 4 displayed the temperature-dependent diameter variation of carbon fiber exposed in air and argon. It can be seen that the diameter kept unchanged in argon.…”

“…[13][14][15] The property degradation of CFRP plates in a fire mainly stems from the oxidation of carbon fiber and the decomposition of resin at elevated temperatures. 16,17 Relevant investigations have shown the high-temperature decomposition of carbon fibers gave rise to the microstructures variation and led to the degradation of the mechanical and thermal properties. 18,19 For example, Liu et al 20 found the elevated temperature brought about the decrease of tensile strength of carbon fiber, which was attributed to the decrease of crystallite size and increase of interlayer spacing.…”