The effect of moisture on a glass/epoxy composite

Abstract

Research was done to determine the effect of moisture on the transverse tensile strength and the interfacial shear strength of a glass/epoxy composite. Specimens with two different fiber sizings, one epoxy compatible and one vinyl-ester compatible, were immersed in distilled water to give a saturation level of moisture absorption. Both aged and unaged samples were tested in two different ways. A new procedure was developed to perform transverse tensile tests of laminate composites in an electron microscope. Using this technique, one can directly observe the failure scenario. Results were analyzed to see if moisture modifies the failure scenario or the ply stress levels at which damage initiates and propagates. Also, micro-indentation tests were done to measure interfacial shear strength. Micro-Indentation results show that the system with the incompatible sizing had a lower interfacial shear strength than the system with the epoxy-compatible sizing. The critical shear interfacial shear stress of both systems was decreased by moisture absorption; however, the weaker interface showed twice as much degradation due to moisture. The decrease in the epoxy compatible system can be explained by changes in residual stresses. However, the decrease in the system with the incompatible sizing is probably due to changes in the residual stresses plus some degradation of the interface. The transverse tensile tests also showed that the system with the incompatible sizing was weaker. Both systems showed little degradation due to moisture absorption. The slight decrease in the critical stress to cause transverse cracking could be explained by relaxation of thermal residual stresses by the absorbed moisture. Insitu observations show that almost all of the failure occurred by debonding at the fiber/matrix interface at heterogeneities in the microstructure which coalesced into transverse cracks.