The damage resistance and tolerance of flat [(0/45),/core/(45/0), ] sandwich plates with honeycomb core subjected to low-velocity impacts using hemispherical steel impactors has been investigated experimentally. The effects of impactor diameter on the impact behavior, resulting impact damage states, and residual strength under in-plane compressive loading was of particular interest. The impact responses characterized in terms of peak impact force was observed to be dependent on the facesheet type, core thickness, and impactor size, but was found to be independent of the boundary support conditions. The smaller impactor produced damage states characterized by residual dent depths that were comparable to the core thickness, accompanied by visible facesheet fractures. The larger diameter impactor produced damage states with large core damage regions but with dent depths less than the facesheet thickness. Under in-plane compressive loading, depending on the impact damage state, contrasting failure mechanisms involving net-section fracture and buckling failure were observed. A reduction in compressive strength up to 60% of the undamaged strength has been observed.
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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