Void formation in composite prepregs – Effect of dissolved moisture

Optimization of curing cycle in carbon fiber-reinforced laminates: Void distribution and mechanical properties

“…Consolidation is normally carried out in an autoclave because the high hydrostatic pressure (>0.7 MPa) ensures full compaction of the laminate, leading to a maximum fiber volume fraction and negligible porosity and, thus, to optimum mechanical properties. Autoclave processing has, however, major drawbacks from the industrial viewpoint (long processing time, limited production rate and component size, high capital investment) and there is an enormous interest in optimizing out-of-autoclave processing routes so high performance composites can be processed at higher rates and lower cost [1][2][3][4][5].…”

“…The stability of voids as a function of the temperature and pressure has been extensively studied by Kardos et al [9], who considered the effects of the resin viscosity and of the resin-void surface tension. They developed a model for void growth which was successfully applied to predicting the occurrence of voids in thermoset composite materials manufactured by liquid molding [10], autoclave or vacuum bagging [3], Although these models provide the essentials of the mechanics of void growth in a polymer blend, they are restricted to small spherical voids surrounded by a viscous resin. Their validity is not proven for long cylindrical voids in a viscous matrix surrounded by aligned fibers, the standard morphology observed in out-of-autoclave composites.…”

Effect of laminate edge conditions on the formation of microvoids in composite laminates

“…From the images, it is found that the number of small intra-tow voids (i.e., voids <50 μm) present in the laminate increased with flow restriction. This is not surprising since the outflow of resin from a laminate is known to be one of the predominant paths for which mobile voids, such as the intra-tow voids observed in these specimen, are removed [10,16,17]. Thus, one can deduce from the experimental results that increasing the resin flow restriction made the escape of the intra-tow voids less likely and, as a result, the number of voids trapped within the laminate increased.…”

“…The moisture present in the prepreg was expected to facilitate the formation of voids, as articulated in detail in Refs. [10,15], and allow for the effect of resin flow restriction for laminates fabricated at low pressures to be more clearly identified. A cure cycle of a 1°C/min ramp from 25°C to 121°C, followed by a two-hour isothermal hold at 121°C, and a 10°C/min ramp down to 60°C was used for all laminates.…”

“…<2%) are most often manufactured by the autoclave process [6][7][8]. Yet the composites industry is increasingly considering lower cost alternative processes such as hot-press molding because autoclave curing is expensive partially due to the high cost of the required equipment [8][9][10][11]. However, before hot-press molding or other out-of-autoclave processes can be implemented as an alternative to autoclave curing, techniques must be developed to allow for components to be fabricated with a comparable high quality.…”