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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Fig. 2. Fraction of fatigue cracks along the basal plane plotted against the angle between notch alignment and crack growth direction.

3.2. Crack tip deformation and crack growth mechanism Figure 3(a) shows a schematic diagram that shows mode II fatigue crack growth along the basal plane. Figure 3(b) shows an example of SEM images that indicates a micronotch corresponding to the same crystallographic orientation relationship between the notch tip and basal plane, as in the schematic illustration shown in Fig. 3(c). A distinct slip line appeared in front of the crack tip in Fig. 3(c). Subsequently, the fatigue crack further propagated along the slip line, as shown in Fig. 3(d). The slip line/band is a precursor of fatigue cracking along the basal plane. More specifically, the shear-stress-induced local slip near the crack tip causes damage accumulation on the basal plane, which causes further propagation. In addition, crack growth paths are not greatly influenced by the presence of the β phase. To obtain more details on the mode II crack growth, the crack tip deformation was characterized by an in situ observation during one cycle at 2501 cycles, as shown in Fig. 4. The crack tip region is sheared without crack opening, which is evidence of mode II crack tip deformation. Repetition of the mode II crack tip deformation induces an intensive slip line, which causes mode II fatigue crack growth, as shown in Fig. 3.

Fig. 3. (a) Schematic diagram of crack tip and crystal orientation; (b) SEM image of FIB notch before test; (c) SEM image of crack tip at N = 1000; (d) SEM image of crack tip at N = 1500 — the orange line indicates the basal plane determined by EBSD. 3.3. 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