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.

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.

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