Investigation of Residual Strength and Fatigue Life of Unstiffined Aluminum Panels with Multiple Site Damage.

Abstract : Multiple Site Damage (MSD) is the occurrence of small fatigue cracks at several sites within aircraft structures. This is important since it may lower the residual strength and fatigue life of the structure beyond what can be predicted using the damage tolerance technique based on a single crack, currently in use to design aircraft structures. This study investigated the effects of MSD on unstiffined panels. MSD usually occurs at rivet holes, or other stress concentration locations within an aircraft structure. This study simulated rivet holes with MSD, by using holes of constant diameter with small cracks, evenly spaced across the midspan of specimens. The objective of the study was to test the validity of the available analytical methods to predict the residual strength and fatigue life of panels with MSD. Residual strengths of large specimens with MSD were measured in two different configurations to test the applicability of five failure criteria. A total of ten residual Fatigue tests were also conducted in three configurations to test the ability of analytical methods to predict fatigue life. The analytical methods investigated in this study can provide an aircraft designer with a conservative estimate of the residual strength of aircraft structures with MSD damage. However, the three failure criteria considered in this study produced unconservative fatigue life predictions. (MM)