Abstract: Based on Paris and Walker's forulas two expressions are presented of fatiguc crack propagation in all area, which both consider the effects of stress-ratio and thrcshold value,and give different rules of fatigue crack propagation in different stages.Besides incorporating demands of damage equivalence,they are able to estimate service life and maintenance cycle of the structure affected by low load,of which the damage is not to be ignored.So they are very worthwhile in cnginccring.

Key words: fatigue, crack propagation, st ressratio