Moisture effects on the behavior of graphite/polyimide composites

The effect of moisture on the material behavior of Magnamite(R) IM7 Graphite/Avimid(R) K3B thermoplastic polyimide composite laminates has been investigated. Laminates consisting of a 62 vol% fiber were laid up with several different stacking sequences; [90](10) or unidirectional, [O-2/90](s) or thin cross-ply, [O-2/90(2)/O-2](s) or thick cross-ply, [45/O/-45/90](s) or quasi-isotropic, and [O/90] laminates. In this study, the glass-transition temperature, T-g, and the intralaminar fracture toughness, G(1c), were measured for dry and moisture-saturated unidirectional samples, When laminates were saturated with moisture, the value of T-g was found to decrease from its baseline (dry) value, but recovered upon redrying the samples. This observation is consistent with the effects of moisture on the T-g of other polymer composites. Permanent toughness losses have not been observed in samples conditioned in room-temperature 75% and 100% relative humidity environments. However, during long-term conditioning of cross-ply and quasi-isotropic samples in liquid water, transverse cracks initiated in the absence of an applied mechanical load. Moisture uptake curves for conditioning in room-temperature liquid water, 80 degrees C [176 degrees F] liquid water, and at room temperature and 75% relative humidity were used to calculate Henry's Law constants and diffusion coefficients. Non-Fickian behavior, consisting of a postsaturation increase in moisture uptake, was observed in cross; ply and quasi-isotropic laminates and might be due to the observed transverse cracking.