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40 W. HU AND S. NUTT Figure 2. Schematic of in-plane permeability test set-up. The perforated resin film was fabricated by punching holes 2 mm apart using a coring tool with diameter 0.25mm. Prepreg plies were cut to 127mm 127mm, while the perforated resin film was 38mm 38mm. Each laminate consisted of 4 prepreg plies and was cured in a programmable air- circulating oven (Thermal Products Solution Blue M). A 4-hour debulk at room temperature and 60C were performed for both PW and UD lami- nates. Temperature was measured by two thermo- couples on the glass plate side throughout the cure cycle. Time-lapse videos were recorded throughout the cure using a portable microscope (Dino-Lite Premier 2 Digital Microscope) with a magnification of 20. Void content of the resin-rich prepreg surface as a function of time was measured in situ for each test panel. Six representative images were selected from the time-lapse video for subsequent analysis for each test. Regions where the prepreg was not in contact with the perforated resin film were consid- ered as voids. Voids were manually selected and converted to binary, and void content and size were calculated using image analysis software (ImageJ). Void content was determined as the ratio of the area of voids to the total area. 2.3. Permeability measurements Gas flow can be used to characterize permeability of a porous medium. For laminar flow, the relationship between the flow rate and permeability is commonly expressed by 1-D Darcy’s law [10, 12]: Q 1⁄4 KA dP (1) l dx where K is the permeability scalar in the flow direc- tion, A is the cross-sectional area of the sample, P is pressure, x is distance, and m is gas viscosity. 2.3.1. In-plane permeability test In-plane permeability was measured using a steady- state air flow test. According to Equation (1), gas permeability can be measured by applying a constant flow rate of fluid through a porous medium and measuring the pressure drop across the length of the sample. For compressible gas, assum- ing the gas follows the ideal gas law, the flow rate at constant temperature can be described as [18]: KA P2P12 Q1⁄42Ll P (2) 1 where P1 is the pressure at the vent side, P2 is the pressure at the vacuum side, and L is the sam- ple length. The experimental set-up is shown in Figure 2 based on the work by Arafath et al. [11, 12]. The laminate was bagged with three isolated compart- ments. The right compartment (vacuum side) was used to apply vacuum to the laminate, while the left compartment (vent side) was connected to a mass flow rate controller (MC-10SCCM, Alicat Scientific) to allow air flow into the laminate. The center com- partment contained the laminate, which was sealed on all sides with sealant tape, allowing flow only in the in-plane direction. In this way, by imposing a known volumetric flow rate using the flow rate con- troller and measuring the pressure on both sides, permeability was calculated using Equation (2). During the pre-cure heated dwell, resin viscosity drops, and resin gradually infiltrates the dry fiber tows. Consequently, a steady state flow cannot be achieved, hampering measurement of permeability. Thus, rather than attempting to measure the in- plane permeability during dwell, laminates were first partially processed to different degrees of impregna- tion according to the tow impregnation model developed by Centea et al. [17], then cooled to room temperature at each point of interest to avoid further resin flow. Permeability measurements were then conducted on such laminates at room tempera- ture. The length and width of samples were L1⁄450.8mm and W1⁄4101.6mm, while the thickness of each sample was measured before each perme- ability test using a micrometer. For UD prepregs, each sample consisted of 12 plies of prepreg stacked [0]12, while for PW prepregs, each sample consisted of 8 plies stacked [0/90]4s. To obtain an average effective permeability value, three samples (repli- cates) were measured for each degree of impregna- tion with a minimum of three flow rate trials per sample. 2.3.2. Through-thickness permeability test Through-thickness permeability was measured using a “falling pressure” method, which is frequently used in cases of low gas permeability where the steady-state flow test cannot be applied. A custom test fixture, shown schematically in Figure 3, was used for the experiments based on the work ofPDF Image | VACUUM BAGGING SUPPLIES Peel Ply Fabric
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