Objectives:

To verify the fatigue disbond propagation models developed for constant amplitude mode I stress state to a block loading. The material is woven-roving glass/vinyl ester composite

Approach:

Previously we developed two fatigue disbond propagation models for the opening mode stress-state under constant amplitude displacement loading (Fig. a). The models covered all three regimes of fatigue propagation, namely, threshold, Paris’ linear, and unstable regimes. The two equations best described the experimental data plotted in log-log and log-linear scales. The two equations are verified for an aggressive block loading here(GImax=0.5GIC-2.0GIC). The block loading test was performed by stepped (interval 10 k cycles) constant amplitude displacement loading. Figure b shows the intended and actual loading in terms of mode I energy release rate(GImax) normalized by mode I fracture toughness (GIC=1.97 lb/in). For each interval of loading, there are three GImax values associated with initial, final, and average crack lengths.

Results:

Figure c shows a comparison of two equations with experimental data for 80 k cycles. All computations were done for GImax (average). Equation (1) predicted the crack growth fairly well (Initial crack length is 1.45 in; measured final crack length is 4.99 in and predicted final crack length is 9.49 in) while Equation (2) predicted a much longer crack propagation.