Mechanical testing of carbon-carbon composite Facesheet (1)

A summary of the mechanicaltest results is shown in Table IV. Also included in Table IV are data for an 8-ply material that was produced by BFGoodrich using the same fibers, fabric, matrix and heat treatments as the material in this study. The major difference between the two composites was the number of plies and the directional lay-up.

The average tensile strength was 16.7 ksi and the average modulus was 12.6 Msi. The tensile strength and modulus were the only properties measured in this study that were significantly different from projected by the trade study. In addition, the tensile data had a large amound of scatter. One possibility for the low strength is that the short length of the fibers in the gage section, maximum of 1.3 inches, is affecting the measured strengths and moduli values. Although short effecitve fiber length might give a lower average strength, the large amount of scatter was not anticipated. Future testing of specimens cut in the 22.5° direction should help to clarify the issue.

The average compression strength value for the 0° direction was 14.3 ksi and 13.6 ksi for the 90° direction. The compression strength projected in the trade study was 13 ksi, very close to the measured values. The compression modulus values were 14.7 Msi in the 0° direction and 12.8 Msi in the 90° direction. The measured compression modulus is slightly below that projected in the trade study, 16 Msi. However, because of the difficulty of trying to attach the clip gage to a very thin specimen edge and the short gage length, the scatter in the data was higher than usual. Due to the high scatter, a statistical test was run to determine if there was a significant difference in the modulus values measured between the 0° and 90° directions and among the 0°, 90° and the projected values. The statistical test showed no significant difference between the modulus values for the 0° and 90° directions at an alpha of 0.05. There was also found to be no significant difference between the measured modulus value for the 0° direction and the projected value of 16 Msi at an alpha of 0.05. However, there was found to be a difference between the modulus value for the 90° direction and the projected value at an alpha of 0.05 but not at an alpha of 0.025.

The interlaminar shear strength was measured using a double-notch shear specimen. The initial trial sample failed in compression at the thin web of material next to the gage section. After this experience, tabs were glued on the remainder of the test specimens to carry the load into the shear section. This resulted in four successful shear failure tests. In the majority of the other cases the shear failure occured between the plastic tab and the C-C composite. The average interlaminar shear strength value was 1.4 ksi. This is a good value for most types of C-C composites but below the 2.5 ksi typically measured for CVI-densified C-C.

The interlaminar tensile strength was measured using a flatwise-tensile test using 0.75-inch square specimens. All specimens failed between the two C-C plies. The average ILT strength value was 1.3 ksi. This value is higher than was measured for the 8-ply composite and may be related to betterpenetration of the CVI matrix due to the thinness of the composite.

The in-plane shear strength was measured using the v-notch Iosipescu method. The average in-plane shear strength value measured was 13.7 ksi. Due to the thin nature of the composite it was found to be necessary to tab the specimens. The average shear strength value is significantly greater than the 10 ksi measured for the 8-ply material and the 9.5 ksi predicted by the trade study. The reason for this is probably due to the fact that all the fibers help to resist the load applied to the specimen due to the 22.5° angle of the lay-up. In typical quasi-isotropic lay-ups tested in the 0° direction some of the fibers would have been parallel to the shear direction and would have provided no reinforcement.

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