A distribution function of two dimensional threshold is given for fatigue crack propagation.A two dimensional stress threshold interference model is set up.The model is applicable to the reliability analysis and design of infinite life for propeller blades or other structural members with initial defects and accident damage.The functional correlation between the reliability of no crack growth and the initial crack sizes is obtained from the model and the maximum permissible crack sizes can be determined according to a given reliability level.In consideration of the fact

that the longer the service time

the greater the probability of accident damage

the overhaul periods of propeller blades with high reliability are also given.