GAS FLOW PATHS IN CARBON/POLYMER COMPOSITESSUBJECT TO LN2 AND COMBINED LN2 AND 177 °C CYCLING

摘要

The distribution of ply-level micro-cracks in carbon/polymer composites was determined asa function of cryogenic cycling. IM7 / 977-2 carbon/epoxy, IM7/5250-4 carbon/bismaleimide, andIM7/3K e-beam cured carbon/epoxy cross-ply ([0/90]2S) and quasi-isotropic ([0/45/-45/90]S) laminateswere tested for up to 1000 cycles. The thermal cycles consisted of either submersion in liquid nitrogen(LN2) or a combined cycle consisting of equilibrating periods in LN2, at room temperature, and at 177 °C.Several interesting trends were noticeable. Reducing the ply thickness by 30% in the IM7 / 5250-4delayed surface ply micro-cracking by up to 200 cycles, but by 1000 cycles the surface ply micro-crackdensities were nearly equal regardless of the ply thickness. Sectioning and polishing revealed greatermicro-crack densities 2.5 cm away from the sample edges than at the edges several plies of each laminate.Finally, the addition of a hold of 5 minutes per cycle at the maximum service temperature (177 °C)increased the micro-crack density in all plies of the IM7 / 5250-4 laminates a minimum of 2 times and by400 cycles had produced micro-cracks in every ply of the [0/+45/-45/90]S laminate.