Cryogenic/elevated temperature cycling induced leakage paths in PMCs

Three structural carbon/polymer composites (IM7/977-2, IM7/977-3, and IM7/5250-4) were thermally cycled between liquid nitrogen (LN2) temperature and an elevated temperature of 120 or 177 °C. The extent of ply-level micro-cracks was measured as a function of cycles up to 1000 cycles as one indicator of suitability for cryogenic containment applications. The choice of material systems, lay-ups, and thermal cycles allowed the study of the effects of the thermal cycle profile, mode I toughness, and cure temperature on the damage accumulation. The addition of a hold period at elevated temperature led to micro-crack initiation after fewer cycles and increased the micro-crack density in all plies for all of the material systems. The consequences of adding the hold period at elevated temperature to the thermal cycle were particularly severe for IM7/977-2. The IM7/977-2 samples were nearly micro-crack free after thermally cycling between room temperature and LN2 temperature, but had up to 64 times greater micro-crack density in some plies after being subject to cycling between elevated temperature and LN2 temperature.