Many engineering components work under combined torsion and axial loading in their working conditions and the cyclic combined loading can result in fatigue fracture after a very long life fatigue regime. Thus, the investigation extends a wider range of test conditions involving surface treatment and manufacturing effect such as machining, so as to understand the fatigue properties and damage mechanisms of the material beyond 10~9 cycles. This work reviews the effect of surface conditions on the fatigue behaviour of mechanical components in Gigacycle regime. Evidently, surface conditions can be variable and due to the very different sources such as manufacturing effect like machining or final surface processes on the parts, heat treatment before and after manufacturing or environmental conditions like corrosion. In fact this is a detail comparative study based on the experimental results carried out by our research team work in this domain. Experimental investigation on the test specimens tested at a frequency of 20 kHz with different stress ratio R=-1 at room temperature. All of the fatigue tests were carried out up to 10~(10) cycles. Damage mechanism was evaluated by Scanning Electron Microscopy (SEM).
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