Editing Damage tolerance (section)

==Safe-life structure==

Not all structure must demonstrate detectable crack propagation to ensure safety of operation.  Some structures operate under the [[safe-life design]] principle, where an extremely low level of [[risk]] is accepted through a combination of testing and analysis that the part will never form a detectable crack due to fatigue during the service life of the part.  This is achieved through a significant reduction of stresses below the typical fatigue capability of the part.  Safe-life structures are employed when the cost or infeasibility of inspections outweighs the weight penalty and development costs associated with safe-life structures.<ref name="riddick"/>  An example of a safe-life component is the [[helicopter rotor]] blade.  Due to the extremely large numbers of cycles endured by the rotating component, an undetectable crack may grow to a critical length in a single flight and before the aircraft lands, result in a catastrophic failure that regular maintenance could not have prevented.