© 2013 Siemens Energy, Inc. All rights reserved. and (h)). This type of barrier is very common for most the specimens in all three orientations, and it is supposed that the effect of this kind of barrier on crack growth is much more important than the crystal boundaries. As KΔ arises, extensive crystal facet formations have been found for all three orientations. The reasons may be, as KΔ become so large that the local barriers can hardly produce any effective resistance, thus a predominant crack can easily and quickly travel though a crystal grain, and the appearance of crystal facet is attributed to the need for rapid energy dissipation by the advancing crack. Such energy dissipation is the fastest when cracks travel along the easiest path, i.e., the slip facets of each grain. Figure 9 shows the typical claw-like barriers that have been widely found in the crack paths. It is suggested that when crack tip starts to reach these local barriers, it has been stopped due to the existence of these barriers, and as cyclic loading going on, new cracks start to re-initiate from these barriers; Subsequently, as the new cracks keeps growing and finally coalesce with the original crack, these barriers are seen to be passed through. Therefore it is inferred that these claw-like barriers do play an important role in cracks growth, and sometimes the effect of these barriers are so huge that the role of grain boundaries can hardly be seen. Besides, worth mentioning that in the previous pre-test procedure where some same C(T) specimens made of CM 247LC DS were conducted FCG tests, very few or no claw-like barriers had been found in their crack paths, so those specimens showed significant orientation dependence, and explanations associated with grain boundaries’ resistance were made and it seemed that these explanation did work. As a result, there is reason to believe that due to the existence of numerous claw-like barriers in all three orientations, crack growth behaviors show weak orientation dependence for this DS material. Figure 9. Typical claw-like barriers in the crack path
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