The history, logic and uses of the Equivalent Initial Flaw Size approach to total fatigue life prediction

Total fatigue life is traditionally composed of the time to crack initiation plus the time for the initiated crack to grow to a critical crack size. Fracture mechanics does reasonably well in predicting the growth portion but there is still a lot of uncertainty about the definition of an initiated crack and scatter associated with the number of cycles to “initiation”. This paper will review some of the history, logic and uses of the Equivalent Initial Flaw Size (EIFS) approach to total life prediction. In short, this is a method where found cracks are analytically grown backwards to time equal zero (time or cycles) to determine an initial flaw, referred to as an EIFS. By growing a number of found cracks back to time equal zero a distribution of EIFS can be established. Example of establishing this distribution are given for the C-130 aircraft with a 7075 aluminum structure and for gas powered turbine blades made of directional solidified super-alloys.