6F-Polyimides with a phenylethynyl endcap (HFPE) were fabricated into carbon fiber composites using the standard polymerization of monomer reactant (PMR) approach via the ester/acid route. T650-35/HFPE carbon fiber composites were evaluated against the corresponding T650-35/PMR-II-50 composites with nadic endcap at 315ï370 C (600ï700 F) for their physical and mechanical properties as well as thermo-oxidative stability. In addition, concentrated HFPE monomer solutions were infused into stitched AS4 and T650-35 preforms and their mechanical properties were compared with those of the commercial BMI-5270 composites from cryogenic temperature to 343 C (650 F). The stitched HFPE composites out-performed BMI-5270 composites in terms of property retention at elevated temperature and microcrack resistance during thermal cycling from ï54 to 288 C (ï65 to 550 F). The stitched composites also showed more resistance toward blistering and delamination than conventional laminate composites during a rapid heating rate simulating launch and re-entry of reusable launch vehicles (RLV).
An experimental procedure for measuring interlaminar shear strength and fracture toughness in composite laminates was proposed. The procedure was used to determine the interlaminar properties of a quasi-isotropic laminate of IM7/K3B thermoplastic composite that had undergone thermal or thermo-mechanical aging. A five-point bending device and the ENF fixture were used to determine the interlaminar shear strength and interlaminar toughness, respectively. The aging effects on these two properties of IM7/K3B under various aging conditions were evaluated. It was found that the aging effect on interlaminar toughness is similar to that on interlaminar shear strength.
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