Fast cures for thick laminated organic matrix composites

Abstract

A quick cure schedule is presented for a 1-in. thick laminated graphite—epoxy composite. The heatup time required to reach the epoxy reaction temperature can be reduced to 60 min from over 300 min. A numerical optimization scheme based on a process model that accounts for the type of tooling is used to calculate the time-optimal cure schedule. During the begining of the cure schedule, the autoclave temperature is raised by 15°C above the epoxy reaction temperature of 175°C. At the onset of the reaction, when the laminate temperature is 75°C, the autoclave temperature is reduced rapidly. This cooling removes just enough of the heat of reaction so that the laminate temperature continues to increase, but in a controlled manner. When the laminate temperature reaches the epoxy reaction temperature and the reaction starts to slow, the autoclave temperature is raised to match that of the laminate. The model is shown to accurately predict the unsteady temperature distribution through the thickness during cure. The physical constraints of the processing equipment, such as maximum heating and cooling rates and maximum and minimum temperatures, are included in the optimization scheme. A Crank—Nicolson-type finite-difference scheme that employs an iterative calculation at each time step is shown to be only slightly more accurate than a similar scheme in which the iterative calculation is not performed.