Environmental Durability of Graphite/Epoxy Composites: The Combined Effects of Moisture, Cathodic Polarization, and Stress

About the report:

The increasing acceptance and incorporation of fiber-reinforced polymer matrix composites (PMCs) as engineering construction materials have led many to look to the infrastructure as an application for these versatile materials. One such system is pultruded graphite fiber-reinforced epoxy (graphite/epoxy). Some PMC systems degrade when subjected to environmental conditions (e.g., moisture, stress, UV light, electrochemical polarization). These variables are typically studied either singularly or in series, but in real applications (e.g., aerospace, marine, infrastructure), these materials are subjected to many of these conditions simultaneously. To simulate field conditions, this study investigated the combined effects of an aqueous environment, electrochemical polarization, and applied bending stress on the durability of a pultruded graphite/epoxy composite. The findings indicate that graphite/epoxy composites cannot be assumed to be insensitive to degradation by environmental variables. Further, electrochemical polarization, as might occur with contact with a metal such as a fastener, can accelerate degradation. This damage requires the presence of moisture. Chloride and sulfate concentrations in rain are sufficient to establish an electrolyte within creviced regions, but deicing salts would overtake these as a contributor to conductivity. Further findings may be summarized as follows: 1.) Application of polarization in an aerated 0.6M NaCl environment led to breakdown of the fiber/matrix interface. The high pH environment created during the oxygen reduction reaction was necessary but not sufficient to create this breakdown, as the unpolarized specimen exposed to a pH 13 environment did not degrade. Cathodic polarization as would occur by coupling to steel or aluminum is required. 2.) Application of cathodic polarization did not significantly alter strength. Average measurements of shear strength, however, did decrease with the application of cathodic polarization for 70 and 90 days.

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