Onset of multiple site damage and widespread fatigue damage in aging airplanes

This paper presents a methodology for predicting the thresholds of multiple site damage and widespread fatigue damage in fuselage lap slices. Widespread fatigue damage is a type of multiple cracking that reduces the airframe residual strength to a level below the damage tolerant requirement. The MSD threshold refers to the point in the lifetime of an airplane when two adjacent collinear fatigue cracks can linkup at the allowable stress. The WFD threshold is the point in time when linkup of a primary crack created from accidental damage and secondary cracks created from fatigue can result in a catastrophic failure. The methodology presented in this paper combines results from residual strength analysis and fatigue crack growth testing to determine these thresholds. In particular, a displacement compatibility approach is adopted to calculate residual strength in curved stiffened panels tested in the laboratory. The laboratory experiments also include fatigue testing of full-scale panels containing a debonded lap slice. Based on this methodology, the threshold of widespread fatigue damage for these laboratory panels, adjusted to zero minimum-stress cycling, is between 32,000 and 40,000 cycles, and the threshold of multiple site damage is about 70,000 cycles.