Effect of an External Stress on Moisture Diffusion and Degradation in a Graphite-Reinforced Epoxy Laminate

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Abstract
The effect of external stress on moisture diffusion and mechanical properties degradation of a graphite/epoxy composite, Scotchply SP-313, has been investigated. Moisture gain as a function of time is determined for the composite subjected to four tensile stress levels in the range of 0 to 65 percent of its ultimate tensile strength (UTS). Experimental techniques to determine moisture gain in a composite subjected to external stress have been developed and discussed. In addition, the residual (after drying) and the current (wet) short-beam shear strength and the flexural strength as a function of moisture content and external stress level have been determined for the composite. The formation of environmental damage by the combined stress and humid environment conditions as well as the mode of failure in the short-beam test was investigated by scanning electron microscope (SEM) studies. Results indicated that the moisture absorption process into the graphite/epoxy composite subjected to external stress, at the levels used in this study, can be predicted effectively by Fick's second law of diffusion. Increasing the external stress level produces higher effective diffusivities, and higher initial absorption rates. A significant increase was observed in laminates subjected to more than 45 percent of their UTS and containing more than 1 percent moisture. This was attributed to early formation of cracks within the 90-deg plies caused by moisture degradation of the matrix or interface strength. The current and residual short-beam shear strength was reduced with increase in moisture content. By enhancing the moisture penetration into the composite, the external stress accelerates the mechanical degradation process. The flexural strength, which is a fiber-dominated property, showed a very low sensitivity to application of external stress for the levels used in the present study.

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