SAMPE Journal - March/April 2011 - 56

Figure 13. Initial tension- tension fatigue results. (R=0.1, 10Hz) (1ksi= 6.9 MPa).

Figure 14. Tension-tension fatigue data of panels fabricated simultaneously with varying flow pathway sizes and pattern. (R=0.1, 10Hz).

edge specimens, which were of higher quality due to the race tracking that occurred during the infiltrations, were the better performing specimens and fell well within the stress concentration factor, Kt=2 and Kt=4, data of 2024 aluminum18. (Kt is a measure of the effect of geometry on stress at a notch. A Kt=1 specimen is designed to give pristine material behavior with no significant stress concentrations. A hole in a plate generally has a stress concentration of Kt=3 if it is not too close to the edge of the material and is not large relative to specimen dimensions.) The Kt =2 and Kt =4 curves for notched aluminum provide data for comparison to that of the FML fabricated herein. Therefore, the better quality samples appear to provide a reasonable fatigue response as compared to notched aluminum. The smaller pattern spacing and the larger holes allows for better wet out and quality while the smaller holes should provide improved fatigue life compared to the larger holes. The larger panels (40 cm x 40 cm) fabricated with the smaller hole pattern with both the larger holes and smaller holes were tested in fatigue and the data is shown in Figure 15. The data is compared to the results shown in Figure 14 with the poor quality specimen data removed. (Average values are shown for clarity.) The high quality panels with the larger holes and small spacing (0.81mm/ 1.27cm) and with the small holes and small spacing (0.41mm/ 1.27cm-2), resulted in improved fatigue properties compared to the previous panels especially at the lower stress levels. Since these panels were fabricated with thicker Al foils, the percentage of composite is lower compared to the initial data obtained at Georgia Tech from Reference 14 and 15. It is evident from the data that the smaller holes resulted in better fatigue life, as expected, while providing adequate flow to produce quality panels. Titanium / Carbon FML High Temperature VARTM The resultant panel from the initial Ti/ IM7/ PETI-8 infiltration had flexural strengths (ASTM D-790) of 1.4±0.12
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GPa at RT and 1.05±0.05 GPa at 177°C. Although the panel appeared to wet out well, photomicrographs (Figure 16) indicated porosity between tows in the fabric layers. This porosity problem has been observed in HT-VARTM composites as well12. Although this porosity issue has been essentially corrected in carbon fiber composites, it is apparent that further refinement of the processing conditions for HTVARTMFML will be required. The larger HT-VARTMFML 20 cm x 27 cm panel (Figure 7), however, did not fully infuse from the Al porous flow medium into the Ti/IM7-6k uniweave preform. The similarities in flow observed in the epoxy panels with both the large and small holes does not appear to translate to the polyimide resin system despite similar viscosities at infusion. Future work will attempt to understand the issues associated with HTVARTMFML by developing a more accurate flow model of the process with a specific PETI-8 resin model.

Figure 15. Tension-tension fatigue data of panels fabricated with varying flow pathway sizes and pattern. (R=0.1, 10Hz).
SAMPE Journal, Volume 47, No. 2, March/April 2011

SAMPE Journal - March/April 2011

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