Influence of elevated temperature on the microhardness and microstructure of carbon fiber reinforced polymers

Carbon fiber-reinforced polymer (CFRP) has attracted attention for the reinforcement and retrofitting of concrete structures. Thus, their degradation in elevated temperatures is important feature to investigate. Herein, influence of elevated temperatures on microstructure and microhardness properties of different types of CFRP was investigated. A series of CFRP plate-, sheet-, and bar-type samples were fired at 150, 250, 350, and 450°C. Crystal structural properties of epoxy resin matrices and carbon fibers in the CFRP composites were examined using X-ray diffraction analysis. An increase in surface defects was observed for the fired CFRP plates and bars; however, there was no significant difference between the microstructures of the fired CFRP sheets. Hardness of CFRP plates fired at 150 and 250°C substantially decreased from 0.67 to 0.62 and 0.47 GPa, respectively, compared to that of unfired CFRP plate. Similarly, hardness of CFRP bars fired at 150, 250, and 350°C decreased from 2.09 to 1.73, 1.63, and 0.62 GPa, which is reduced by 17, 22, and 70%, respectively, compared to that of unfired CFRP bar. Decrease in hardness is attributed to several factors, such as mechanical degradation in the resin matrix, fiber/matrix debonding, and an increase in microcracks on the surface.