Effect of humidity on the failure of ethylene vinyl acetate/soda lime glass interfaces using small tensile specimens

The vulnerability of adhesive joints and surface coatings under stress to environmental attack by water continues to be of considerable interest. Using tensile specimens of small dimension and minimizing exposure time we are able to maximize the importance of vapor phase attack of the bond line and to essentially eliminate bulk diffusion. We report simultaneous video observations and load measurements during failure of an ethylene vinyl acetate adhesive on soda lime glass. Samples with one millimeter diameter attachment areas were mounted on a microscope stage in a small, controlled environment chamber, allowing for rapid changes in humidity. The time required for the pre-existing defects to grow to detectable size (10 μm diameter) was a strong function of applied stress and humidity, consistent with chemically enhanced crack growth initiated at a pre-existing defect. Subsequent, visible crack growth showed a much weaker dependence on applied stress and humidity, consistent with growth limited by the diffusion of water vapor to the crack tip. The results are analyzed in terms of a chemical kinetics model of water vapor-enhanced crack growth. We also explore the possible existence of thresholds in stress and/or humidity.