Effects of hygrothermal aging on the fatigue behavior of two toughened epoxy adhesives

Aged open-faced asymmetric double cantilever beam (ADCB) specimens made with two different rubber-toughened epoxy adhesives were subject to cyclic loading under mixed-mode conditions. The contrasting results illustrated the effects of environmental degradation on the matrix and toughener. The fatigue threshold strain energy release rate, Gth, and the crack growth rates of adhesive 1 degraded in two stages: Gth initially decreased with aging time until it reached a constant minimum value at long aging times. Similarly, fatigue crack growth rates initially increased with aging time until reaching a limiting upper value. However, Gth reached the minimum value sooner than did the crack growth rate. In contrast, the Gth of adhesive 2 decreased significantly with aging while the crack growth rates remained unchanged even after prolonged aging. These differences in fatigue threshold and crack growth rate behavior were attributed to changes in the size of the plastic zone at the crack tip as the applied loads changed. The differences in the degradation behavior of both adhesives were explained using gravimetric and dynamic mechanical thermal analysis (DMTA). The degradation of the toughening mechanism of adhesive 1 was related to retained, bound water disrupting the chemical bonds at the rubber/matrix interface. Having no retained water, adhesive 2 was unaffected by this degradation.