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Figure 3. Schematic of through-thickness permeability test set-up. Tavares et al. [10, 15]. A reservoir with a known volume was added to the set-up. When vacuum is applied, the reservoir pressure will decrease if flow occurs through the porous media, provided no leaks are present within the fixture. Using Equation (1), the 1-D laminar flow of compressible air through a porous medium at isothermal conditions is described by (20 C, 40 C, 50 C, 60 C, and 70 C) for four hours, and temperature was monitored throughout the tests using thermocouples. 2.4. Degree of impregnation To determine the degree of impregnation for the in- plane permeability samples, a coupon was processed alongside each sample. The coupons were removed from the oven with the samples and quenched to room temperature to prevent further resin flow. The partially cured laminates were then cold-cured in an ammonia environment at room temperature for a week (following the protocol described by Howard [19]. Ammonia vapor reacts with epoxy at room temperature as a curing agent to achieve a hard and stiff structure while preserving the morphology of the laminate at each point of interest [6]. Samples were sectioned at the center, polished, and inspected using a stereo microscope (Keyence VH-Z100R). 3. Results and discussion 3.1. Inter-ply air evacuation The effects of debulk temperature on inter-ply air evacuation are shown in Figure 4. Initially, air was trapped both in the artificial pores and between the perforated resin film and the first prepreg ply (Figure 4(a) white regions). These bubbles were situ- ated primarily in the gaps between the resin film and depressed regions of fabric due to the natural fiber crimp of PW prepreg. Once vacuum was applied, bubbles disappeared rapidly through the pinholes at the intersections of warp and weft tows. After a 4-hour debulk, the inter-ply air was com- pletely evacuated, both at 20 C and at 60 C (Figures (b,c)). In UD prepregs (Figure 4(d)), the air naturally trapped between resin film and the first prepreg ply (the white regions) was randomly distributed, but was much less prevalent than in PW laminates. After debulk for 4h at room temperature (RT), the white regions decreased slightly in size, while the larger artificial pores did not change in both size and position (Figure 4(e)). However, after a 4-hour debulk at 60C, most naturally trapped air bubbles had disappeared, and some of the artificial air bub- bles decreased in size, indicating that debulk at 60C was more effective in reducing inter-ply por- osity in UD prepregs. Figure 5a shows void content as a function of time in PW prepreg. The initial void content was $35% due to the naturally trapped air. Though the air evacuation rate was slightly lower during the RT vacuum hold, in both cases, void content decreased sharply to $0.3% within the first hour. UD prepregs " # KAPBag PCoreð0Þ þ PBag PCoreðtÞPBag LlV t1⁄4ln P ð0ÞP P ðtÞþP Core Core Bag Core Bag (3) where PBag is the pressure at the bag side, PCore is the pressure at the honeycomb core side, t is time, and VCore is the volume of the core (6.37 104 m3), L is the thickness of the laminate, which was measured after each permeability test using a micrometer. The air viscosity m is a function of tem- perature, which can be updated using the following equation: T0 þCT3=2 T þ C T 5 Prepregs were cut to 127 mm 127 mm and were placed over the reservoir, supported by aluminum honeycomb core. All four edges of the samples were sealed with sealant tape to prevent in-plane gas flow, and a perforated release film was placed on top of the sample to allow air evacuation only in the through-thickness direction, followed by breather cloth and vacuum bag. Vacuum was drawn in the bag to compact the laminates and create a pressure difference between core and bag. The evo- lution of pressure in the cavity was monitored over time using a pressure transducer (Omegadyne, Inc) and data acquisition software (LabView, National Instruments). Measurements were recorded for 4-, 8-, 16-, and 24-ply PW laminates and 1-, 2-, 4-, and 8-ply UD laminates at isothermal temperatures l 1⁄4 l0 (4) ADVANCED MANUFACTURING: POLYMER & COMPOSITES SCIENCE 41 wherel0 1⁄41.8310 PasatT0 1⁄4293K,andCis the Sutherland Constant, 117K. Plotting the right- hand side versus time yields a straight-line plot, the slope of which can be used to determine the effect- ive transverse permeability of the prepreg. 0PDF Image | VACUUM BAGGING SUPPLIES Peel Ply Fabric
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