## Hunting Unlimited 2008 Crack [CRACKED]

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Hunting Unlimited 2008 Crack [CRACKED]

During dynamic loading, the strain hardening and softening behaviour of polycrystalline materials [62] results in microcracking as seen in figure 11. The stresses across a microcrack are typically higher than the stress across an undamaged grain boundary, where the stress intensity factors are evaluated. The highest stress intensities are obtained at the cracks tips where the crack tension is maximal. This non-uniform stress distribution is believed to be the result of non-linear stress-induced grain growth at the crack tip [63]. With increasing number of cycles, the microcrack grows and the remaining material around the crack becomes increasingly refined, resulting in successful crack bridging and hence crack closure. In this process, the remaining virgin material at the crack tip is exposed to lower stress intensities than virgin material in the rest of the matrix. Hence, fatigue crack growth rates are effectively a function of the applied cyclic stress intensity, but also of the remaining virgin volume at the crack tip [64]. As shown in figure 11, crack growth rates are much higher in [65] C36Al than in the present AZ 91 alloy due to the formation of much finer ferrite grains in C36Al, which offers better resistance to crack growth and hence fatigue life.Figure 11. Effect of stress distribution on crack growth rates. After [62]. (Online version in colour.)

The next figures show examples of the interaction between pores and crack initiation by cyclic slip localization. Figure 12 shows a fatigue crack initiated during hexagonal phase lamellae settling at the surface due to cyclic slip localization occurring between twinning bands. Figure 13 shows examples of defects at the interface that limit crack propagation. In both cases, the accompanying micrograph shows a suitable FIB section for crack propagation initiation, since the lamellae are removed and the section imaged. The first case (figure 12) is similar to fatigue crack initiation in magnesium alloys where fatigue cracks initiate preferentially at contraction cavities. Localized fatigue cracks are also commonly observed in Pd-based alloys, which favour a low fatigue crack initiation life [66]. In the second case (figure 13), cracks are initiated in the interface at the tip of pores and propagate along different paths. d2c66b5586