Influence of Mechanical Characteristics of Epoxy on the Formability of CFRP Sheets under Different Temperatures

Abstract

Abstract Autoclave-molded CFRP (carbon fiber reinforced plastic) has high tensile strength and thermal resistance. However, the curing process in autoclave molding makes the cycle time extremely long. To shorten the cycle time, various forming methods such as the RTM (resin transfer molding) have been developed. Although the cycle time has been improved with these methods, the strength of the products is inferior to that of autoclave-molded CFRP. To realize high-speed forming while maintaining a strength equivalent to that of autoclave-molded CFRP, press forming is effective. If a practical and reliable press-forming method for autoclave-molded CFRP sheets is realized, the CFRP production process can be divided into a curing process and a forming process. This means that CFRP can be formed by simply performing press forming on prepared CFRP sheets. Excluding the curing process from CFRP forming will shorten the cycle time. Press forming requires plastic deformation, but CFRP has poor deformability at room temperature. Therefore, it has been believed that CFRP is a brittle material that is not suitable for press forming. However, recent experimental research has shown that the press formability of CFRP sheets is improved at 100oC, compared with that at room temperature. This finding indicates the possibility of the press forming of CFRP, but the temperature dependence of the formability of CFRP has yet to be shown. To establish a press-forming method for CFRP, an analytical study of its plastic deformation is essential. To determine the temperature dependence of the formability of CFRP, the softening of epoxy in CFRP at moderate temperatures was focused on in this study. This is because the mechanical characteristics of the resin have a strong effect on the plastic deformation of CFRP. An approximate curve based on the temperature parameters was constructed to express the stress–strain relationship at different forming temperatures. Then, press-forming simulation models were constructed to consider the effect of the mechanical characteristics of epoxy in CFRP. The approximate stress–strain curves at room temperature and 100oC was applied to the model. Using this model, the stress distribution in CFRP sheets formed at each temperature has been shown, and the result clearly demonstrates that the formability of CFRP sheets is enhanced at higher forming temperatures. These results enable the accurate simulation of the deformation of CFRP sheets, which will help establish a press-forming method for autoclave-molded CFRP. Moreover, our simulation models can be applied to the deformation of various materials that contains thermosetting resin.