The main goal of this research is to gain a fundamental understanding of the synergistic mechanisms of degradation for a model E-glass/vinyl-ester composite exposed to humid environments and to compare them to the mechanisms of degradation resulting from water immersion. Moisture sorption kinetics are assessed in terms of structural modification diffusion in order to understand how water sorption phenomena and leaching of low molecular weight species may be responsible for changes in material properties.

Plasticization is identified using dynamic mechanical thermal analysis (DMTA) and is correlated to reversible degradation of the longitudinal tensile strength and short beam shear (SBS) strength. Tensile strength is also seen to decrease as a result of minimally reversible interfacial degradation, also identified through DMTA and SBS testing. Exposure to 18%RH and 50%RH results in material properties which remain within initial scatter except where increases in the glass transition temperature and SBS strength indicate matrix dominated strengthening also identified in material exposed to 99%RH and immersion at elevated temperatures. Tensile, SBS, and DMTA results all reveal degradation of the fiber resulting from exposure to high humidity and immersion environments at elevated temperatures. Scanning electron microscopy confirms the occurrence of interfacial debonding and fiber pitting. In material exposed to 80°C immersion, pitting of the fiber surface was identified at sites adjacent to kaolin clay, a hydrophilic particulate filler commonly used as a lubricant in pultrusion.

Predictive degradation models are applied to tensile strength, SBS strength, and tensile failure strain results for 99%RH and immersion exposures, where irreversible degradation occurred at elevated temperatures. Degradation resulting from exposure to 99%RH and immersion is found to be equivalent. Predictive models show significant scatter based on the inability to isolate specific mechanisms. Further work is indicated in this area to ensure that safety factors are appropriately selected.