PSI - Issue 7

Yuri Kadin et al. / Procedia Structural Integrity 7 (2017) 307–314 Kadin et al. / Structural Integrity Procedia 00 (2017) 000–000

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reached. From that moment the second stage has been started, and after each loading cycle a picture was taken to visualize the fatigue crack propagation up to fracture. Fig. 7b presents the crack size vs. the number of cycles at the second stage, and Figs. 8 demonstrate the SEM micrographs of propagating crack after 5, 6, 7 and 8 cycles of the second stage. Since the lowest force was slightly positive (10N), the loading ratio R = F min / F max in this experiment was approximately equal to 0.1.

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Figure 7: Low cycle fatigue experiment with rough microstructure ceramics. Cyclic force with progressively increasing peak load (a) and crack extension vs. number of cycles during the crack propagation stage (b).

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Figure 8: Crack propagating under cyclic loading. Micrographs (a-d) correspond to the crack propagation after 5, 6, 7 and 8 cycles, respectively. Fatigue crack under higher magnification (e).

The results presented in Fig. 7b indicate extremely low cycle fatigue regime: as soon as the initiated crack was visible, its propagation till unstable fracture took only a few cycles (see again Fig. 7b). Such fast crack propagation