FGA is a nanograin layer of 500–800 nm thick with grain size of 50 nm.
Nanograin layer exists in both sides of fracture surface of FGA region.
Nanograins form at crack wake via contacting between originated crack surfaces.
Nanograins present for negative stress ratio cases, but vanish for R > 0 cases.
Proposed Numerous Cyclic Pressing model explains FGA formation.