PSI - Issue 13

Akira Maenosono et al. / Procedia Structural Integrity 13 (2018) 694–699 Akira Maenosono et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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during unloading, and, thus, the crack propagated via a general repetitive fatigue process along a slip line that is regarded as mode II crack growth. Therefore, the presence of the microstructure boundary is a factor disturbing the crack growth via mode II crack tip deformation.

Fig. 4. SEM images of a crack tip at 2501 cycles — this indicates a mode II displacement at around the crack tip; the orange arrows indicate the same lattice corner; the images were taken at (a) 0 MPa (before loading), (b) 800 MPa (peak stress), and (c) 0 MPa (after unloading).

3.4. Crack growth behavior a cross prior β grain boundary As presented in the previous section, the fatigue crack predominantly propagated along the basal plane via mode II crack tip deformation as long as the crack propagates in the interior of a colony. However, when the crack reached a grain/colony boundary, the crack growth behavior showed a significant variation, as shown in Fig. 5. First, a tiny void formed in front of the crack tip and on the interface between the intergranular α sheet and α grain when the crack tip reached the interface, as in Fig. 5(a). During loading, the fatigue crack propagated to the location of the microvoid, as in Fig. 5(b), and subsequently coalesced with the microvoid, as in Fig. 5(c). Then, the coalesced crack propagated over the grain/colony boundary, as in Fig. 5(e). After this process, the crack closed during unloading, and, thus, the crack propagated via a general repetitive fatigue process along a slip line that is regarded as mode II crack growth. Therefore, the presence of the microstructure boundary is a factor disturbing the crack growth via mode II crack tip deformation.

Fig. 4. SEM images around a crack tip at 2799 cycles: the mages were taken at (a) 0 MPa (before loading); (b) 400 MPa; (c) 800 MPa (peak stress); (d) 400 MPa; and(e) 0 MPa (after unloading).

3.5. Scatter of fatigue life associated with mode II crack growth As shown in Section 3.2, the dominant crack growth mechanism was revealed to be a mode II type that requires a significant shear stress on the basal plane. In this section, we suggest some causes of the scatter in fatigue life in terms of the crack growth mechanism. The shear stress along the basal plane at the crack tip depends on the nominal stress and angle between notch/crack alignment direction and basal plane. Therefore, the predictions of fatigue life and its scatter require considerations of the effects of nominal stress and crystallographic orientation. In this regard, the scatter of a Ti – 6Al – 4V alloy with the HCP structure is larger than that of other materials that have many slip systems, such as FCC