Residual stress, generated during the curing process of epoxy and CFRP composite structure, exerts significant influences upon their mechanical properties. The origin of the residual stress can be classified into two groups. One is generated by the volume change during the curing step. Generally curing temperature is higher than the $T_g$(glass transition temperature) of the structure, and the molecules at curing temperature have enough mobility to compensate the volume change. The other is generated by the nonuniform shrinkage during the cooling step. When the structure is cooled from its final curing temperature, the surface region is cooled faster than the center region because of the low thermal conductivity of the structure. Nonuniform temperature distribution causes different density profile along the thickness of the structure. The surface region reaches $T_g$ first and becomes glassy state where molecules are immobilized. As the cooling is continued, molecules at the center region also become glassy state which accompanies volume shrinkage, and since the surface molecules are already immobilized the volume shrinkage at the center part causes residual stress.
In order to reduce the residual stress, many methods are suggested. In this paper, after studying the curing mechanism of the resin and measuring the physical properties, optimum curing process that can minimize the residual stress is investigated.
The reaction kinetic equations were derived for the neat epoxy and CFPR composite system through the dynamic DSC technique. The physical properties such as $T_g$, thermal conductivity, heat capacity, density were measured and their changes with temperature and conversion were expressed. Using these basic equations the conversion, temperature, density profiles were obtained by the computer simulation. To check the validity of the computer simulation program the temperature changes at the center of the structure were measured. The measurement showed that the program simulated the curing process quite well.
No residual stress was generated during the curing step since the curing temperature was always higher than the $T_g$. Almost all the residual stress were generated during the cooling step. To quantify the residual stress, the area under the specific volume distribution fuction was considered as the relative residual stress when the surface temperature reached the $T_g$ of the structure. By varying the final curing temperature and cooling rate, the relative residual stress were calculated. The residual stress decreased with lower curing temperature and slower cooling rate.
