p. 366-371
Effects of small defects on the fatigue strength of Ti-6Al-4V alloy were investigated by tension-compression fatigue tests. The artificial holes having 50, 200, 400 and 1000μm diameters were introduced onto the surface of a series of specimens (Series A). Another series of specimens (Series B) were prepared to investigate the effect of a burr or pre-crack, which was introduced at the edge of the artificial hole. The fatigue limit was defined by two ways depending on the threshold conditions for Series A and B: (1) the threshold for crack initiation from the edge of the hole for series A, and (2) the threshold for crack propagation from the burr of the hole or the pre-crack tip for Series B. As a result of the fatigue tests, the fatigue limits of Series A specimens were 20〜60MPa higher than that of Series B. This is because the crack initiation stress of titanium alloys is very high but once a crack initiates at the edge of a hole, the nonpropagating crack is hard to exist in the microstructure of Ti-6Al-4V, Therefore, the √<area> parameter model underestimated the fatigue limit of Series A but it predicted accurately the fatigue limit defined by the crack-growth threshold for Series B. Besides, the fatigue life was also decreased by the burr. This is because the burr easily became the crack initiation site and reduced the number of cycles to crack initiation.
