FATIGUE-CRACK PROPAGATION AT POLYMER ADHESIVE INTERFACES

Fatigue (slow) crack growth in epoxy/glass, epoxy acrylate/glass and e poxy/PMMA interfaces was studied under constant and cyclic loading at both high and low humidities using the interfacial, four-point flexure test. Finite element analysis was used to determine the energy releas e rate and phase angle appropriate for the different crack geometries observed. The experimental results show that for the polymer/glass int erfaces, the primary driving force for fatigue crack growth is the app lied energy release rate at the crack tip and that increasing test hum idity enhances crack growth under constant loading but has an insignif icant effect under cyclic loading. At low humidity the crack growth ra tes under cyclic loading are significantly greater than under constant loading. For epoxy/PMMA interfaces the crack growth results were inde pendent of the applied energy release rate, relative humidity, and cyc lic rs. constant loading, within experimental scatter. In addition, fo r polymer/glass interfaces the effect of phase angle (13 to 54 degrees ) on crack growth rates is not significant. However, for epoxy/PMMA in terfaces the applied energy release rate for the initiation of crack g rowth is considerably greater for a phase angle of 66 degrees than for 5 degrees, indicating that increasing shear at the crack tip makes th e initiation of crack growth more difficult. These results are discuss ed in terms of possible mechanisms of fatigue crack growth al polymer adhesive interfaces.