Hygrothermal property of domestic carbon fiber/bismaleimide resin composites for aeronautic application

For a kind of domestic T700-grade carbon fiber and four types of bismaleimide resin (QY9611, 5429, QY9512, QY8911-4) used in aeronautic field, three hydrothermal conditions, including 100℃ boiling water, 70℃ water immersion and 70℃/85% relative humidity, were adopted to treat these unidirectional composite laminates. Moisture absorption content, diffusion coefficient, microstructure, chemical groups, resistance to temperature and mechanical property were studied to evaluate hydrothermal property of composite. The results show that the moisture absorption of the four types of composites under the three conditions all follows Fick's second law. Moisture equilibrium content and diffusion coefficient are the largest under boiling water condition, while they are the least under the condition of 70℃/85% relative humidity. Obvious differences on the speeds of moisture absorption among the four bismaleimide composites have the relationship with material type and molding method. Hydrothermal treatment does not result in damage inside composite and chemical change, and the resulting plasticization effect results in decrease in glass transition temperature. The test results of 90° tensile properties of the composites show that the coupling of elevated temperature and moisture absorption causes more serious degradation in composite's mechanical property, and its failure mode transforms from matrix cracking to debonding and fracture at interface region.