A method based on two-step investigations has been proposed for the determination of laws of the blade material damage development.

The method has been used for testing turbine blades made of EI-437B material and test specimens of EI-437BWD material. Results of the experiments have been presented in the form of median lifetime values as obtained in the second stage of investigations versus average lifetime values as obtained in the fust stage.

An equation describing the lines of equal damage probabilities has been derived with the use of an appropriate mathematical apparatus. This equation gives good approximation of the experimental results. The lines of equal damage probabilities are quasiperiodical in form, this being due to the fact that the lifetime distribution density function of the objects tested was non-monotonic. The equation of the lifetime distribution function has been presented. 

The mathematical model developed has been based on the assumption that the process of changes in the state of an object (due to the damage) is random and occurs in increments, the damage accumulation varying with the time.

The obtained model of the distribution, confirmed expefimentally can be used when forecasting the turbine blade lifetime.