Constant- and variable-amplitude fatigue tests were conducted on 7075-T6 aluminum alloy built-up beam specimens in bending to study certain aspects of spectrum fatigue. Existing cumulative fatigue damage theories were found inadequate for the dual purpose of predicting spectrum fatigue life and evaluating the effects of individual load levels in a spectrum. However, the observed spectrum fatigue properties were consistent with the measured effects of preloading, overloading, and underloading.
Preloading was beneficial when the preload was considerably greater than the subsequent fatigue loads, and when the preload was applied in the same direction as the fatigue loads. When the preload and the fatigue loads had opposite signs, a reduction in fatigue life resulted. These effects were magnified by the periodic application of overloads.
The periodic application of repeated low loads was ineffectual when applied in the same direction as the fatigue loads. When the spectrum included low loads of negative sign, serious reductions in life resulted.
The most valuable finding, perhaps, was that the fatigue behavior of the beam specimens was similar to that of certain full-scale aircraft structures. It was also found that the fatigue properties of the beam specimens were rather insensitive to variations in testing technique and material.
A pneumatic testing machine and an automatic programmer that were developed for this investigation are described.