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 the manufacturing process or in service. Defect, 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 fatigue and very high cycle fatigue (VHCF) behavior of selective laser melting Ti-6Al-4V alloys. The S-N curve descends approximately linearly for internal defect induced failure. While for artificial surface defect induced failure, the S-N curve descends at first and then exhibits a plateau region feature. The competition of interior crack initiation with fine granular area feature is also observed in VHCF regime. The paper indicates that only the size or the stress in-tensity factor range of the defect is not an appropriate parameter for describing the effect of defect on fatigue crack initiation. Finally, the effect of artificial surface defect on high cycle fatigue and VHCF strength is modeled, i.e., the fatigue strength sigma, fatigue life N and defect size root area(square(root of the projection area of defect perpendicular to principal stress direction) is expressed as sigma = {CNa(root area)(n,) N < N-0 CN0a(root area)N-n,N->= N-0 where C, a and n are constants, and N-0 is the number of cycles at the knee point of the curve.