13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- 4. Results and discussion The fatigue crack growth experiment was ended by the action of the displacement limiter at N = 2.08 × 106 cycles. A fatigue crack that nucleated and grew at the root of the cantilever caused the limiter action. Figure 7 shows the displacement range (Δδ) of the load piston during the fatigue experiment. Until N = 1.0 × 106 cycles, the displacement range maintained a constant value. However, at N = 2.0 × 106 cycles, the value rose slightly and then suddenly began to increase. This sudden increase was thought to indicate the fatigue crack growth at the root of the cantilever. Therefore, in this case, the intended fatigue crack growth experiment was performed until around N = 1.0 × 106 cycles. 4.1. Mode II fatigue crack growth behavior Figure 8 shows the relationship between the potential difference and the number of cycles measured using the AC potential method. In this experiment, because the effect of electric noise was not removed, averaged data were considered. From Fig. 8, it can be seen that the crack began to grow from N = 5 × 103 cycles, after which the crack growth depended on the number of cycles. 4.2. Fracture surface of Mode II fatigue crack After the fatigue crack growth experiment, the fracture surface was observed using a scanning electron microscope (SEM). Figure 9 shows the result of this observation. In this figure, lines that were parallel to the crack growth direction and cracks branching in the Mode I direction were found, which are characteristic of a Mode II fatigue fracture surface [3, 4]. Therefore, the success of the Mode II fatigue crack growth experiment was verified. (a) (b) Figure 5. Models for derivation of friction: (a) mass-spring model and (b) load-displacement curve Figure 6. Estimated load-strain curve
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