possible that external pressure applied on the vacuum bag was insufficient to remove the

bubbles.

 After cryogenic cycling on the specimen C2, microcracks were observed in the

90-degree layer as shown in Figure 4-9. The fiber breaks were not observed in the in 0-

degree layer were not observed since the thermal stresses were not large enough. The

delaminations propagated along the middle of the 90-degree layer where some voids were

found. The transverse cracks branched with the delaminations. Since the gas can be

transmitted through the transverse cracks across the specimen, the permeability increased

after cryogenic cycling.




















 0.05 mm

Figure 4-9. Microcrack propagation on the graphite/epoxy composite specimen C2 after
 cryogenic cycling: (a) 10X magnification; (2) 40X magnification.

 For the textile composite specimen T1 before cryogenic cycling, no microcracks

were observed as shown in Figure 4-10. Voids were observed at the location where two

yarns are merging.