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Controlling voids to minimize the final porosity level is an important concern when processing advanced composite structures. In this study, the porosity evolution during processing of partially impregnated prepregs is investigated using interrupted cure cycles and optical microscopy. Laminates made of MTM 45-1/5HS carbon/epoxy prepreg subjected to different cure cycles, bagging conditions, and humidity levels were studied. Fiber tow geometry and gas permeability were measured to determine the amount of compaction and the interconnectivity of unsaturated zones in the laminates. Three types of voids were identified: inter-laminar, fiber tow and resin voids, all with different origins and evolution patterns. It is shown that gas transport (both in-plane and through-thickness), fiber bed compaction, and resin infiltration govern void evolution during processing. The results provide insights for development of representative transport models and to optimize processing cycles. [ABSTRACT FROM AUTHOR]

Partially-impregnated prepregs are three-phase systems consisting of fiber, resin, and gas. During processing, one of the main goals is to evacuate the gas and infiltrate the space left behind with liquid resin to minimize porosity. Most work in this area has focused on gas and resin transport independently, and the interaction between resin and gas transport in these systems is currently poorly understood. The current study examines this interaction by evaluating the resulting laminate porosity as a function of different bag and applied pressures during debulk and cure. Experimental results show that resin flow can promote gas evacuation without the need for hard vacuum under the bag. An explicitly-coupled gas–resin transport model is developed on the basis that fiber-bed porosity is a function of resin infiltration. The model extends previous work in the area and is shown to predict the experimentally measured porosity under different pressure situations with acceptable accuracy. [ABSTRACT FROM AUTHOR]

Void characterization and porosity measurements of uncured and partially cured carbon/epoxy prepregs are challenging due to the soft nature of the matrix. If samples need to be cut from a larger laminate, the act of cutting and polishing can alter the void morphology. This paper presents a method to prepare samples for optical microscopy by infiltrating the pore space in the soft prepreg with a room-temperature curing low-viscosity resin to support the structure during cutting and polishing in preparation for optical microscopy. The methodology is validated by comparison with results obtained from porosity measurements using the ASTM D2734 standard density method. The paper also explores the use of thickness measurements to determine porosity. It is shown that thickness measurements can be used to estimate porosity for the no-bleed out-of-autoclave prepreg system used in the present study but that the accuracy is lower than using microscopy or density methods. [ABSTRACT FROM AUTHOR]