Past experience with tests of structures under simulated flight loading has indicated that the time to initiation of cracks from most structural details such as sharp corners or holes is relatively short and that the majority of the life (i.e., 95%) is spent growing the resultant cracks to failure.  Likewise, analyses of in-service fractures, cracking instances, etc. have indicated that a major source of cracks is the occurrence of initial manufacturing defects such as sharp corners, tool marks and the like.  Thus, it is now common practice to consider the damage accumulation process as entirely crack growth, with zero time to initiate the crack.  Although this assumption may seem unduly severe, recent studies have shown the approach feasible, of minimal detriment to weight, cost, etc., but most important, the consideration of initial damage in the form of cracks or equivalent damage is absolutely necessary to ensure structural safety.

This subsection will detail the fundamentals of life prediction based on crack growth.  The crack length will be the measure of damage and the crack growth rate will define the rate of damage accumulation.