Measuring the negative pressure during processing of advanced composites

Pre-impregnated sheets of advanced carbon fibers with highly viscous thermoset-based resin systems are commonly used in fabricating aerospace composite parts. As prepreg sheets are vacuum bagged and heated during processing, the resin slowly infiltrates in dry spots between fibers. Under vacuum condition, surface tension-induced capillary effects impose deep negative pressures on the liquid resin, leading to tensile stretching. A method is presented to quantify the capillary-induced negative pressure. During processing, samples are quenched to freeze the infiltration process before taking SEM images. Using image analysis and by invoking interfacial stress boundary condition, the pressure distribution is determined. Sub-micron radii of curvature at the resin interface were measured corresponding to local negative pressures up to several atmospheres and an average pressure of - 10 to -50 kPa. Effects of negative pressures on the state of material and on defects such as resin phase separation are discussed.