UBC Theses and Dissertations
Gas transport and void evolution in composite prepregs Kay, James
Out-of-autoclave (OOA) processing of prepregs represents a promising low cost alternative to expensive autoclave processing, which today is the standard method for producing high-quality CFRP parts such as aerospace structures; however, OOA processes have a greater potential to experience problems with voids than autoclave processes do. Understanding the evolution of voids during the prepreg process, and the OOA prepreg process in particular, is a key to robust and low cost processing that can produce high quality composite parts with low porosity. The evolution of voids in prepregs is best understood as the result of interacting transport mechanisms during the debulk and cure processing steps. The most important such mechanisms are the in-plane flow of gasses through an interconnected network of voids, the diffusion of dissolved moisture within the resin, the sorption and desorption of moisture into and out of the resin, and the flow of resin into the void spaces. A large experimental project has been carried out to experimentally measure the effect of varying the parameters of the OOA process on the porosity levels of cured parts. The results of the experiments show that factors like part size, moisture content, and debulk time influence the porosity levels of parts produced. The governing transport mechanisms have been investigated and characterized, and a numerical model capable of simulating the simultaneous action of these interacting mechanisms is proposed. The numerical model was validated by comparison with experimental measurements of moisture content during a debulk, and it was shown that the proposed model is able to predict the measured values with reasonable accuracy. A method of estimating part porosity using the numerical model was proposed and its results were compared with the experimental results. The knowledge presented is then used to explain observed experimental results published by other authors and to suggest a novel processing technique.
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