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

The fire resistance of carbon fiber reinforced polymer (CFRP) composite is a huge challenge for strengthening and repairing structures in civil engineering. In the present study, the effects of temperature exposure on the mechanical, thermal and microstructure properties of carbon fibers (~700°C) and CFRP plates (~300°C) with the bisphenol‐A epoxy matrix (E51) and hydantoin epoxy matrix (HY) were investigated. The thermal decomposition, void content, surface morphology, and internal microstructures were obtained to reveal the degradation mechanism of CFRP plates. Exposure at elevated temperatures brought about the obvious degradation of the tensile strength of carbon fiber and CFRP plate, and the final tensile strength retention rates of CFRP plates were 72.6% with the low glass transition temperature of CFRP (E51) and 72.3% with the high glass transition temperature of CFRP (HY), respectively. The allowable maximum exposure temperature was estimated to be 200°C to guarantee the desirable tensile stiffness for the CFPR plates. Oxidation of carbon fiber has an inhibition effect on the decomposition of E51 resin in CFRP (E51). By comparison, the decomposition of HY resin and oxidation of carbon fibers were independent in CFRP (HY). The increased void content of CFRP plates at elevated temperatures aggravated the debonding of fiber/resin interface and weakened the synchronous load‐bearing capacity between the fibers, which led to the decrease of mechanical properties. Finally, the rule‐of‐mixture and the elastic mechanics theory were applied to obtain the life prediction model of tensile strength of CFRP plate at elevated temperatures. According to the model, the limit exposure temperatures of tensile strength were 483°C for CFRP (E51) and 466°C for CFRP (HY), respectively.