UBC Theses and Dissertations
Void evolution during processing of out-of-autoclave prepreg laminates Farhang, Leyla
Out-of-Autoclave (OOA) prepreg processing is a promising candidate for replacement of autoclave (AC) processing, which is the current standard for manufacturing primary structural parts in the aerospace industry. However, its success is dependent on the ability to produce high quality parts with low porosity. This thesis develops an understanding of porosity in this process by studying the evolution of the voids during processing. Characterization of voids in partially cured laminates is challenging due to the soft nature of the prepreg matrix. A method for preparation of partially cured samples for optical microscopy and porosity measurement is developed and validated by comparison with results from the ASTM standard density method. It is also shown that thickness can be used to estimate porosity for the no-bleed prepreg system used in this study but that the accuracy is lower than microscopy and density methods. The evolution of voids during different processing cycles and process conditions was studied using the aforementioned optical microscopy on partially cured laminates made of MTM 45-1/5HS carbon/epoxy prepreg. Fiber tow geometry and gas permeability were also measured to determine fibre tow compaction and the gas transport capability throughout the cure process. It is shown that gas transport, fiber bed compaction and resin infiltration govern void evolution during the process. Porosity is governed by multiple chemical and transport phenomena, among which gas transport through vacuum evacuation plays a crucial role. An understanding of gas transport in OOA prepreg processing is developed by examining the time scales for gas transport by Darcy flow and molecular diffusion and comparing those to experimental gas permeability and porosity data. Darcy flow is shown to be the primary means of gas removal during the process. The study shows that the dominant direction of gas transport is dependent on the aspect ratio of the laminate, the prepreg material and the processing history as both in-plane and through-thickness permeability vary throughout the cure cycle. Based on these observations, a simple debulk map that gives the minimum recommended room temperature debulk time for OOA laminates as a function of in-plane and through-thickness dimensions is presented.
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Attribution-NonCommercial-NoDerivs 2.5 Canada