Characterization of Moisture Absorption and Its Influence on Composite Structures

Abstract

This work characterizes moisture absorption and its influence on elastic moduli for various composites. Three thermoset-matrix composites and one thermoplasticmatrix composite were examined. The first was a traditional carbon/epoxy composite; the second was similar except that the epoxy was a polyurethane-modified type. The third was an interleaved composite, adding a Nylon 6 non-woven fabric in between the carbon/epoxy plies. The last was a carbon/Nylon 6 composite made from commingled yarns. Composite panels with various fiber orientations were fabricated for material characterization. Based on these materials, prototypes of tennis rackets were fabricated for studying the loss of stiffness due to moisture absorption. Fabrication of the panels and rackets has been discussed. Two hygrothermal environments have been applied to the rackets; one is a normal condition and the other is a wet-and-hot condition. Histories of moisture absorption and the induced softening effect have been studied. Two standard tests of the racket flexibility have been adopted for measuring the loss of stiffness. The coefficients of hygroscopic diffusion were calculated for each material using the Fickian type of moisture diffusion. Compared with the thermoset composites, the carbon/Nylon 6 absorbed much more moisture and resulted in a much greater reduction in stiffness. Because of the non-woven layers, the interleaved composite was more prone to moisture absorption than the two epoxy-based composites. The relationship between moisture content and stiffness loss differs greatly for the material systems under different conditions.