crack tip closure concept

measurment of FCG based on crack tip closure concept depend on :

1- experimental technique

2-location of the measurement

3- mean stress effect on FG at high stress ratio can not be explained by the closure model due to the absense of the crack closure at high loads.

Two question can be investigated as follows:

1- why crack tip closure is popular and it widely use in hightech industry like aircraft industry acording to overwiew of scentific articles.?

2-basically if we can define clearly residual stress in plasic and cyclic zone mathematically not physically what will happen in developing of crack tip closure concept?

Thank you,

Hadi