THE EFFECT OF THE MOISTURE ABSORPTION ON THE INTERFACIAL STRENGTH OF POLYMERIC MATRIX COMPOSITES

Seven composite material systems have been studied to determine their potential suitability for structural applications for continuous immersion in sea water. The matrices of these composites have been found to absorb moisture with saturation occurring at 0.6%-2% of the matrix weight of additional moisture over approximately 3% present after fabrication, when soaked at ambient temperature in simulated sea water, with 20.7 MPa (3000 p.s.i.) hydrostatic pressure giving a very minor increase in moisture absorption. Pure water absorption gave a slightly higher saturation level than did simulated sea water. With the exception of the graphite/vinylester composites, the degradation in transverse tensile strength and interfacial shear strength due to moisture absorption has been found to vary from 0%-22%, with the thermoset/graphite systems and the vinylester/gIass systems both showing sufficient promise to justify further study. The observed correlation in the decrease in interfacial shear strength due to moisture absorption with decreases in transverse tensile strength supports the hypothesis that the moisture-induced degradation is associated with a decrease in the interfacial strength rather than the degradation of matrix mechanical properties. In situ fracture observations in the scanning electron microscope further support this hypothesis.