Interior initiation and early growth of very high cycle fatigue crack in an additively manufactured Ti-alloy

Very high cycle fatigue of an additively manufactured Ti-6Al-4V is studied at R =-1, 0.1 and 0.2. The fracture surface of crack initiation and early growth region presents fine granular area (FGA) morphology with discon-tinuous regions of nanograins. The equivalent crack growth rate in FGA is in the magnitude of 10(-13)-10(-11) m/cyc based on the "tree ring" patterns marked on fracture surface. Interior crack initiation and early growth is attributed to the cracks caused by nanograins formed during fatigue loading and the cracks formed irrespective of nanograins. The crack growth rate in FGA is also used to predict the fatigue life.