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Experimental study of the pre-gelation behaviour of composite prepreg Duffner, Caitlin


As the composite industry grows, the science base that supports the industry must also grow. In the past, most of the work done on thermoset composite prepreg systems have studied the post –gelation response, leaving the initial condition of the material relatively unexplored. The initial, or pre-gelation, state of the material is becoming increasingly relevant as the industry transitions to new manufacturing processes, mainly automatic fibre placement (AFP) and forming, that handles and manipulates the raw prepreg. Recent works investigating manufacturing defects in composite parts have shown a strong connection between the initial condition of the material and the final part quality. As research and industry shift to working with raw prepreg, it is time to re-examine the pre-gelation behaviour in order to improve processes in the future. This work studies the pre-gelation behaviour of unidirectional Hexcel AS4/8552 prepreg through several novel test methods. The methods presented offer a different perspective into the behaviour of the material. Initially a microscopic perspective was taken, and SEM microscopy was used to characterize the initial morphology of a ply of prepreg. This study presents the significance of variations in morphology and the impact of capillary flow driven ply consolidation. Next, to evaluate the macroscopic behaviour of prepreg, a non-contact approach using Digital Image Correlation (DIC) was developed. This method was able to capture global and local strain responses in the prepreg throughout processing. Findings show that the pre-gelation response is not as simple as previously thought. The material experiences a strong consolidating force when heated due to the effects of surface tension. Further tests performed in this study attempt to make connections between local morphology, resin variations and local variations in strain. These tests used DIC, IR thermography, optical microscopy, and SEM. New insights into prepreg conditions are gained by evaluating the common trends seen with this suite of testing. This work concludes that pre-gelation behaviour in prepreg is a highly complex state dependent on the variability of the material and the liquid-solid interfaces in the ply, as opposed to the traditional approach which considers the material response through a solid mechanics lens. Supplementary materials available at: http://hdl.handle.net/2429/73441.

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