A detailed ultrasonic cycle fatigue study has been presented for aluminum alloy 7075-T6 (AA7075-T6) under fully reversed tension-compression loading to investigate the geometric sensitivity, frequency effects, size effects, surface roughness effects, and the corresponding failure mechanisms across different fatigue regimes. The dominating sources of life variability were identified using sensitivity studies conducted using finite-element analysis (FEA)-based modal and harmonic analyses and probability studies for typical ultrasonic fatigue specimens. In contrast to some studies of crack initiation for aluminum alloys under fatigue, subsurface crack initiations were observed for low stress amplitudes. The presence of both surface and subsurface crack initiations was strongly correlated to the surface conditions and loading conditions. Fatigue-fracture behavior was evaluated via detailed scanning electron microscopy (SEM) analysis of fracture surfaces, exhibiting unique features that are not observed under conventional fatigue testing of aluminum alloys.
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