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44 W. HU AND S. NUTT Figure 7. Transverse permeability as a function of tempera- ture for UD laminates. transverse permeability decreased by an order of magnitude in the first hour, then increased slightly for 4-ply and 8-ply laminates, while for 16-ply and 24-ply laminates, the permeability fluctuated during the remainder of the debulk (Figure 6(b)). As resin viscosity decreases and resin mobility increases, new air pathways can form, provided the gas pressure gradient overcomes the resin resistance. As ply number increases, creating continuous flow channels become more difficult, decreasing perme- ability. Delay time is another indication of this pro- cess. Delay time refers to the time between the start of vacuum hold and the onset of the pressure decrease in the honeycomb core [10]. A delay in pressure drop in PW permeability tests was observed only at 40 C. For an 8-ply laminate, the average delay time was $6min, while for a 24-ply laminates, the delay time increased to $32 min. When resin viscosity decreased to $1500Pas dur- ing the 50 C debulk, the permeability of PW prepreg increased slightly for all cases, but continued to decrease with increasing ply number (thickness), as shown in Figure 6c. However, during 60 C and 70 C dwells, the permeability of 4-, 8-, and 16-ply laminates increased and stabilized at a similar value ($21014 m2) throughout the dwell. In contrast, the 24-ply lami- nates exhibited a permeability value one order of mag- nitude lower. This finding, while somewhat unexpected, can be understood because when resin viscosity is sufficiently low (<1000 Pas), the resistance of resin to air flow decreases to a minimum, and when a pressure gradient is present, air evacuation pathways can be easily created. Air permeability is independent of number of plies within a thickness range (<$4mm). However, as ply number increases, the driving force for air evacuation (pressure change per thickness) decreases, and air evacuation pathways become more tortuous, causing air permeability even- tually to decrease. The effects of debulk temperature on transverse permeability of UD prepegs exhibited clear differences from PW prepregs. First, because each pressure decay test required 30 min to several hours for UD laminates due to low permeability, the trans- verse permeability was plotted as a function of tem- perature rather than time (Figure 7). UD laminates exhibited much lower permeability than PW lami- nates. For a 1-ply laminate, the average permeability at room temperature was 1.11017 m2, $3 orders of magnitude less than that of a 4-ply PW laminate. As the number of UD plies increased, transverse permeability decreased sharply. For a 2-ply and 4-ply laminate, the initial permeability at room temperature decreased to 4.8E-19 m2 and 1.0E-19 m2, respectively, two orders of magnitude lower than that of the 1-ply laminate. For an 8-ply laminate, permeability was effectively zero (undetectable). The sharp decrease in gas permeability with increasing number of plies occurred because gas flow was dominated by a few resin-starved areas, and as ply number increased, these air pathways were occluded. Permeability increased with increasing temperature in all cases. However, as ply number increased, a higher debulk temperature was required to achieve an increase in permeability. The air permeability began to increase when the debulk temperature reached 60C for a 4-ply laminate, while for an 8-ply laminate, no pressure drop was observed until the temperature reached 70C. The increase in transverse permeability facilitated through-thickness gas evacuation, and con- tributed to the more efficient inter-ply air evacuation described in Section 3.1. 3.3. In-plane permeability during pre-cure dwell Figure 8a shows the in-plane permeability as a func- tion of degree of impregnation (DOI). The initial in- plane permeability of PW laminates ($6 1014 m2 with DOI 1⁄4 0.25) was slightly greater than that of UD laminates ($2 1014 m2 with a DOI 1⁄4 0.3). However, for both PW and UD laminates, in-plane permeability decreased continuously with increasing degree of impregnation, as resin infiltrated and occluded gas evacuation pathways. When the degree of impregnation reached $0.8, the in-plane permeabil- ity decreased to 2.3 1015 and 3.7 1016 m2, for PW and UD, respectively, $1-2 orders of magnitude less than the initial state. The permeability of fully impregnated laminates was also measured, although a steady state was not achieved with the lowest flow rate, indicating a permeability less than 1 1018 m2. The permeability data were then correlated to predictions of a tow impregnation model developed by Centea et al. [17, 20]. The model was used to generate in-plane permeability during dwell for both PW and UD prepregs (Figures 8(b,c)). During the RT vacuum hold, the laminate exhibited the greatestPDF Image | VACUUM BAGGING SUPPLIES Peel Ply Fabric
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