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Rates of fatigue crack growth were measured in fatigue tests of stiffened panels constructed with bolted and integral stringers. The panels with bolted stringers were made from 2024-T3 aluminum alloy sheet with either aluminum alloy or steel stringers. The stringers were attached to the sheet with interference fit lock bolts. Stringer spacing and stiffness were varied systematically in the construction of the panels. The integrally stiffened panels were made from 7075-T6 aluminum alloy sheet extruded with outstanding stringers.
The stress intensity factor, calculated by a previously developed method, is used to predict the crack growth rates for the stiffened panels. Fatigue tests were conducted on unstiffened panels to determine the relationship between the stress intensity factor and crack growth rate.
In general, the stress intensity factor correctly predicts the crack growth rates in panels with bolted and with integral stringers except when the cracks are long. In these cases, the measured rates are slightly higher than the predicted rates. Furthermore, the stress intensity factor correctly predicts the rates to be lower in the panels with stiffer and more closely spaced stringers and to be equal in panels with steel and with aluminum alloy stringers of equal stiffness. The bolted stringers reduced the crack growth rate significantly below that for an equally stressed unstiffened panel, whereas the integral stringers had no significant effect.
aircraft, aircraft panels, joists, stiffening, fatigue (materials), cracking (fracturing), crack propagation, cyclic loads, axial stress, stress cycles, tensile properties, inspection, fatigue tests, aluminum alloys, structural steels
Aerospace technologist, Langley Research Center, National Aeronautics and Space Administration, Hampton, Va.