So it has been a while now that I am trying to understand the concept of size effect seen in fatigue of metals.
As far as I have understood(if correctly!), size effect in metals is more due to a fractal nature, rather than energetical because the fracture process zone is small in metals. I have seen also papers which use fractal theories to account for it.
I have seen a good deal of research done by Bazant for concrete and quasi-brittle materials, but somehow I cannot make a proper analogy for metals.