Real-time control and optimization of curing in thick sectioned thermoset composites

Thick sectioned composites are traditionally manufactured using the same Manufacturer-Recommended cure Cycles (MRC) as for thin sectioned composites. However, for thick-sectioned composites, significant exotherms occur during resin polymerization, causing non-uniform residual stresses through the thickness of the composite part. This research developed a comprehensive methodology for actively controlling the exotherm within the part, and for maintaining temperature uniformity through the thickness of the part. Material characterization using Differential Scanning Calorimetry (DSC) provided kinetic parameters for the resin curing process. A detailed thermo-chemical model of the resin curing process based on conservation of energy was developed using finite difference techniques. The detailed model is computationally intensive, hence it cannot be run on-line. Low order reduced models were derived from the detailed models and were used to design a nonlinear dynamic inversion controller. A nonlinear observer was implemented to estimate the cure state in real-time, at different depths in the part. The low order model was also used during curing to simulate the entire cure cycle, and thereby predict the temperature profiles within the part. Using online estimation, control and optimization lends robustness to any disturbances or variations in the process.; The control, estimation and optimization algorithms were successfully demonstrated on a press molding process using glass-epoxy prepreg from Cytec Fiberite. Commercially available thermocouples were mounted on the mold surface. Using the model-based controller, observer and the optimizer, the peak exotherm for a 48-layer thick-sectioned composite part was reduced from 25.55 K to 6.685 K while keeping the total processing time the same as the MRC. Residual strain through the thickness of the composite was characterized using the incremental hole-drilling technique. The residual strain was found to be more uniform at varying depths in thick-sectioned composites, when the on-line control algorithms were employed. The practical, noninvasive and inexpensive approach developed in this research make the proposed approach commercially viable for guaranteeing uniform cure in geometrically complex composites. This research will ensure temperature and cure uniformity during the processing of composite parts, thus improving their service life, while maintaining consistent quality from one part to the next.