Ultrasonic fatigue tests were carried out on the aluminum alloy AISI 6061-T6, presenting different values for the principal surface roughness parameters: Ra, Rq, and Rz. For fatigue life comprised between 3 × 105 ∼ 6 × 106 cycles, crack initiates at the specimen surface induced by stress-concentration and micro-plastic deformation (micro-void coalescence); whereas for the very high cycle fatigue (>107 cycles), the mechanism of crack initiation moves to subsurface or inside the specimen and is associated with internal imperfections such as micro-porosities or nonmetallic inclusions. For the first fatigue life regime, compression residual stresses induced by the work-hardening machining process and the associated micro-plastic deformation are the principal factors controlling the fatigue endurance on this aluminum alloy. Experimental results show that fatigue endurance is higher for the high surface roughness in the 3 × 105 ∼ 6 × 106 cycles of fatigue life, whereas this behavior is inversed in the very high-cycle fatigue regime. These results are analyzed in terms of residual compression stresses induced by the work-hardening machining process, the surface roughness, and the reverse yielding or Bauschinger effect.
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