Smart autoclave processing of thermoset resin matrix composites based on temperature and internal strain monitoring

Abstract

Cure cycle optimization and process control for autoclave cure of thermoset resin matrix composites based on temperature and internal strain monitoring were studied. Cure of thermoset resin is usually an exothermic reaction, which causes temperature increase of composites during cure. Slowing down the temperature ramp rate is effective in lowering the peak temperature. However, the slower is the ramp rate, the longer the cure time becomes. Therefore, it is desirable to control the ramp rate in order to depress the peak temperature with prolongation of cure time minimized. Besides that, precise determination of cure completion is also required in order to minimize cure time. The procedure for smart processing described above was developed and tried on laminate of carbon fiber/epoxy resin prepreg. In this procedure, temperature ramp rate is controlled so that the peak temperature predicted by Springer's thermochemical model is kept below an allowable value. Cure completion is determined by a cure rate equation and internal strain monitoring with embedded EFPI optical fiber sensors. The internal strain is correlated with specific volume change of the matrix resin caused by cure shrinkage and thermal expansion/contraction. The authors found that the cure shrinkage terminates at a certain degree of cure, and EFPI sensors can detect this point. Although the degree of cure can be calculated by integrating the cure rate equation along temperature history, errors may be accumulated. Therefore, the degree of cure is corrected and integration of cure rate equation is restarted at the cure shrinkage termination point detected by EFPI sensors. Thus, cure completion is determined precisely. This smart autoclave processing procedure was able to depress the peak temperature and determine the end of cure.