The attached file contains images of polished samples of the 3-D carbon/carbon sidewall material supplied by Hytec to GSFC. Polishing and
photography was done by Debbie Thomas (GSFC Materials Branch) under the direction of Brad Parker (GSFC Materials Branch). The supplied material was
a full-thickness end piece (about 1” x 2” x 0.47”) cut from a flight plate. In the images the entire thickness can be seen. Due to the limited size of
the supplied sample, no mechanical tests could be performed on the material. The images clearly show a layered structure, with fiber reinforcement primarily in plane. Sections were cut at different angles to reveal fiber orientation. From these sections, it appears that the fibers are semi-randomly dispersed in plane. In addition, the images show large resin pockets (light gray areas) as expected. They also show voids not filled by the resin. An estimate of void content was made using an optical imaging technique, which gave the void content as a percentage of image cross-section area. Figures 1, 2, 3 and 4 had 1.3%, 4.6%, 0.2% and 3.8% voids per cross section, respectively. In the sections with the higher voids, the voids are visible to the naked eye.
My conclusion is that the resin impregnation was not only ineffective in filling the voids, but was also inconsistent in filling the voids in the plate from point to point and from top to bottom. A consequence of variability in voids is variability in panel strength, most notably in the interlaminar tension and peel strengths that affect attachment integrity. Based on the observed void variability, it is imperative that all the design allowables be statistically based. For valid allowables, the number of test samples should be large and taken from enough different places in the plate to reflect material variability effects.