Moisture effects on the behavior of graphite/polyimide composites

Abstract The effect of moisture on the material behavior of Magnamite® IM7 Graphite/Avimid® 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, [0₂/90]_S or thin cross‐ply, [0₂/90₂/0₂]_S or thick cross‐ply, [45/0/‐45/90]_S or quasi‐isotropic, and [0/90] laminates. In this study, the glass‐transition temperature, T_g, and the intralaminar fracture toughness, G_IC, 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°C (176°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 crossply and quasi‐isotropic laminates and might be due to the observed transverse cracking.