63


became higher. The use of nano-particles did not improve the permeability of laminated


composites.



 0-- -16-
 Cl C1
 C2 C2
 C3 C3
 -5 5C5 C5
 T1 T1
 1N1 -N1

 -10 -
 E IE -19
 E E
 -o 15 -o
 --- ------- -20

 -201 f
 0 5 10 15 20 -210 5 10 15 20
 Number of Cryogenic Cycling Number of Cryogenic Cycling


Figure 4-7. Logarithm of the permeability for composite specimens with increase of
 cryogenic cycles.

 Optical Microscopic Analysis

 The optical microscopic inspection was performed to evaluate the microcrack


propagation and void content of various composite systems after cryogenic cycling. In

the previous section, the experimental results showed that the permeability increases as


the composite specimen underwent more cryogenic cycles. As the crack density increases,


gas flow becomes easier though the specimen. Therefore, the microcrack propagation is


correlated with cryogenic cycling.


 The specimens were cut through the center using a diamond saw. A LECO


grinder/polisher was used for the sample preparation process. The rough edge through


the center was ground with 600-grit sand paper with water for 30 seconds. The fine


grinding was performed with the 1000-grit and 1500-grit papers for 30 seconds. The


surface of the edge was polished with the 58[tm aluminum oxide powder (A1203-


alumina) dissolved in distilled water. The purpose of the lubricant is to both dissipate the