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A fatigue investigation was performed for constant- and variable-load amplitudes using a box beam fabricated from 7075-T6 aluminum elements. This structure was tested as a simply supported beam under unidirectional bending. The primary objectives were to determine the relative cumulative fatigue damage resulting from the imposition of four flight-loads test spectra and to determine the effects of stress level, block size, and load sequence on life. A full factorial experiment with two replications was used to perform both fixed- and random-sequence tests for the parameters of load spectrum, static margin of safety, and block size.
The beam exhibited fatigue characteristics that correlated well with those of full-scale structures, the beam-fatigue lives appearing as the upper bound of the lives of full-scale structures. When subjected to fixed sequence loading, a significant increase in life was found for an 11 percent increase in static margin of safety and for increases in test-block size. These results were explained on the basis of the compressive residual stresses produced around the fastener holes. The results support the use of the smallest feasible block size for the fatigue tests of full-scale structures. Comparisons between fixed- and random-sequence experimental data showed that, in general, the smallest fixed-sequence block size most closely simulated random loading within a block and that the fatigue life under random loading was less than that for fixed-sequence loading.
fatigue (materials), aluminum alloys, metals, box beams, stress level, constant amplitude, amplitude modulation, fixed sequence, random sequence, mechanical properties, tests
Aerospace Engineer, Naval Air Development Center, Warminster, Pa.