Defect induced cracking and modeling of fatigue strength for an additively manufactured Ti-6Al-4V alloy in very high cycle fatigue regime
Additively manufactured (AM) alloy usually inevitably contains defects during manufacturing processor in service. Defects, as a harmful factor, could significantly reduce the fatigue performance of materials. This paper shows that the location and introduced form of defects play an important role in high cycle and very high cycle fatigue (VHCF) behavior of selective laser melting Ti-6Al-4V alloy. The fatigue life descends linearly with stress amplitude for interior defect induced failure. While for artificial surface defect induced failure, the fatigue life descends at first, and then exhibits a plateau region feature. We also observed competition of interior crack initiation with fine granular area feature in VHCF regime. The paper indicates that only the size or the stress intensity factor range of the defect is not an appropriate parameter describing the effect of defect on the fatigue crack initiation. Finally, the effect of artificial surface defect on high cycle and VHCF strength is modeled, i.e. the fatigue strength σ, fatigue life N and defect size ( area) (square root of projection area of defect perpendicular to principal stress direction) is expressed as σ= CN( area) for N0 and σ= CN ( area) for N≥N , where C, a and n are constants, N is the number of cycles at the knee point.