An experimental investigation was conducted of the effect of curing cycle on the development of material properties, residual strains and stresses in composite parts during the resin transfer molding (RTM) process. The material investigated consisted of AS4 carbon fibers as the preform and a three-part epoxy system. Unidirectional and crossply carbon/epoxy laminates were prepared by the RTM process in aluminum molds. Several different curing cycles were designed by changing the peak cure temperature and heat up rate based on a cure kinetic model. Strains were measured in the composite laminates during curing using embedded fiber optic strain sensors and electrical resistance strain gages. It was found that significant strain was developed by interference between composite and mold during cure, resulting in constraint-induced strain. The magnitude of residual stresses was also assessed by measuring the warpage curvature of asymmetric cross ply laminates. The data showed that the residual stress was significantly dependent on the cure cycle and the interference.

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A laser ultrasonic system for real-time cure monitoring of a graphite-epoxy composite is proposed. The system contains an array of fiberized laser ultrasonic sources, and an embedded fiber optic Sagnac ultrasonic sensor, and is integrated with a Resin Transfer Molding machine. The use of an optical switch allows ultrasonic generation at several locations of the composite part. Ultrasound generated by the laser source is transmitted into the composite and is detected by the embedded sensor. The cure state is inferred from measurements of ultrasonic velocity. The results of ultrasonic measurements during manufacturing of a composite specimen are presented. The laser ultrasonic cure monitoring system described in this paper has the ability tooperate in a high temperature and high pressure environment; is small enough to be incorporated into molds; and measures the cure state at several locations within the composite part.

The objective of this study is to investigate the residual stress development during the resin transfer molding process. The analysis was based on the strain measurement tests during the entire cure process of unidirectional carbon fiber–epoxy matrix composite laminate using aluminum mold. The investigation efforts were particularly focused on the verification of the strain development mechanism. Although complex interference between the mold structure and the composite laminate was observed during the process, it was found that the final residual strain of the laminate was induced only by thermal mismatch during the cooldown stage. Based on the observation, the residual stresses were calculated by a viscoelastic classical laminated plate theory. The viscoelastic modulus in the calculations was obtained by a series of stress relaxation tests of the material. The curvatures of [0°/90°] asymmetric laminate were calculated, and the results were compared with the experimental data. The results showed that the residual stress developed by the resin transfer molding could be calculated by considering only the cooldown stage of the process.

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