Environmental and chemical degradation of carbon/epoxy and structural adhesive for aerospace applications: Fickian and anomalous diffusion, Arrhenius kinetics

Polymer-matrix composite structures in service may be affected by different environmental conditions that modify their mechanical performance, thermal stability, and flammability potential. In this study, epoxy-based structural adhesive and carbon/epoxy specimens were degraded at three conditioning temperatures, by water and commercial fluids typical of aerospace operations: jet fuel, anti-icing additive and hydraulic fluid. Two well-established simple diffusion models (Fickian, Langmuir) were used to interpret the mass uptake data and assess the suitability of such models for these tests. Despite limitations inherent in these models, reasonably good fits were obtained for adhesive and carbon/epoxy specimens immersed in water or anti-icing additive. We report the respective strengths of association of the two models with the individual specimens’ data, and boxplots for the diffusivity changes among the three conditioning temperatures. An Arrhenius trend for the diffusivity of adhesive specimens treated in anti-icing shows that this temperature dependence may not be a good approximation for this problem. The degradation of these materials in hydraulic fluid is very complex, as indicated by the extensive swelling and the color change, and these models could not successfully fit the gravimetric data. These new results could aid researchers in multiphysics modeling of degraded composite structures.