Threshold of Multiple Site Damage in Aging Airplanes

Widespread fatigue damage (WFD) is a type of multiple cracking that reduces the airframe residual strength to a level below the damage-tolerant requirement In this paper, a distinction is made between the time when multiple cracks can linkup in operation and the time when multiple cracking becomes widespread fatigue damage. The threshold of multiple site damage (MSD) is defined as the point in the lifetime of an airplane when two adjacent collinear fatigue cracks can linkup at the allowable stress. The threshold of widespread fatigue damage is defined as 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. These two thresholds imply different failure mechanisms and, therefore, assume different initial cracking configurations. This paper presents a methodology for predicting these thresholds in fuselage lap splices. The methodology combines results from residual strength analysis and fatigue crack-growth testing. 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 splice. Based on this methodology, the threshold of widespread fatigue damage for these laboratory panels is between 32,300 and 43,500 cycles, and the threshold of multiple site damage is about 70,600 cycles. Since the MSD threshold is larger than the WKD threshold, when a linkup of MSD-type cracks occurs, a WFD condition may exist. In other words, the linkup can lead to a catastrophic failure. Therefore, the MSD threshold is the WFD threshold resulting from linkup of secondary cracks.