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Difficulties on analysis of fatigue short crack behavior

As an issue far from being settled, the analysis of short cracks has always many difficulties.
1, First of all, which method can be used to describe the short crack behavior?
Potential energy calculations in cracked structural components require global stress/strain analyses in all their points, and such laborious calculations limit the energy release rate parameter as an analytical tool to describe and predict cracks behavior. Anyway, it is usually much more convenient to work with SIF in short crack analytical calculations, e.g. Mr. Topper adds the characteristic short crack length a0 on the crack length a; Mr. McEvily adds 1/2 of the Dugdale plastic zone size to get an effective crack length. However, the question is: As approximated LE parameters, the SIF can be used in the analysis of short cracks? Almost in all the traditional reference book, the limitation is: SIF can describe and predict cracks effects in real cracked structures when pz << (a, w  a, h), herein the a is crack length, w is the specimens width and h is the thickness of specimens. The limitation is based on the analysis of elastic-plastic stresses and strains ahead of the crack tip, as if pz << (a, w  a, h), those EP fields are controlled by a much larger elastic field, which that can be well described by SIF. However, how to define the “<<”?? One time or ten times? Let us back to the case of short cracks, perhaps the only one way is to ask the material whether the method based on SIF can be validated (through the experimental results).
2, Secondly, how to treat the short cracks in the same level of grain size? Obviously, the SIF is no longer usable in such anisotropic condition. How to predict the fatigue life in this phase?
3, Does exist the crack closure effects in the short cracks?
4, 2D short cracks propagation analysis.

I appreciate it if anyone could give some viewpoints or suggestions about it.

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