Fatigue life of nominally identical structures under nominally identical loading conditions can scatter widely. This study has investigated the impact of machining processes on such scatters. After Ti 6Al-4V samples were surface ground and face turned, they were subject to constant amplitude four-point bending fatigue tests under room temperature. The best case scenario of process capability ratios of fatigue for these samples were evaluated with assumed tolerances of fatigue life. Based on these ratios, the numbers of nonconforming parts were estimated. Under the machining conditions from a handbook, up to 39% of samples due to one process are expected to be nonconforming while only up to 0.6% of samples due to the other process are expected to be nonconforming. The ramifications in terms of cost for machining quality control due to the different capability ratios have been discussed. The current findings indicate an urgent need to further the study of this issue in a scientific manner.

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