An aging aircraft accumulates fatigue cracks commonly referred to as multiple site damage (MSD). For ductile materials such as 2024-T3 aluminum,MSD cracks may lower the strength signi cantly below that which is predicted by conventional fracture mechanics or net section yield failure methods. An analytical model generally referred to as the linkup model (or the plastic-zone-touch model) has previously been used to describe the MSD phenomenon. However, the linkup model is only accurate for limited geometric con gurations. Two modi cations to the linkup model were developed through regression analysis of test data obtained from the literature and from experimental results conducted in this investigation. The modi ed models show signi cantly improved correlation with the test data over a wide range of con gurations for at 2024-T3 aluminum panels with MSD at open holes. Nomenclature a = lead crack half-length a n = nominal lead crack half-length c = MSD crack length D = hole diameter L = ligament length = half-length for MSD crack and hole, c + D/ 2 t = panel thickness W = panel width b a = correction to stress intensity of the lead crack, b a /`b W b a /`= correction to stress intensity of the lead crack for the effect of the adjacent MSD crack b b = correction to stress intensity of the adjacent MSD crack for the effect of an open hole b`= correction to stress intensity of the adjacent MSD crack, b`/ a b b Ï (c/`) b`/ a = correction to stress intensity of the adjacent MSD crack for the effect of the lead crack b W = nite-width correction to the stress intensity of the lead crack, Ï [sec(p a/ w )] r c = critical stress for ligament failure based on modi ed linkup r LU = critical stress for ligament failure based on linkup, r ys Ï [2L / (ab 2 a +`b 2 )]r Test = critical stress for ligament failure obtained from testing r ys = yield strength
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