Fatigue Strength of Ti-6Al-4V Alloys Containing Small Artificial Defects

The effects of small defects on the fatigue strength of Ti-6Al-4V were investigated in tension-compression fatigue tests. To simulate the defects, holes having 50, 200, 400, 760 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 a pre-crack which was introduced at the edge of the artificial hole. For Series A specimens, the fatigue limit was defined as the threshold for crack initiation from the edge of the hole. For Series B specimens, the fatigue limit was defined as the threshold for crack propagation from the burr or the tip of the pre-crack. The results of the fatigue tests for R=-1 indicated that the fatigue limits of Series B specimens were 20∼60MPa lower than those of Series A specimens. In contrast to steels, non-propagating cracks did not form at the holes, a fact associated with the relatively high stress level for crack initiation from hole in Ti-6Al-4V. It was also found that the area parameter model underestimated the fatigue limit of Series A specimens, but the model accurately predicted the fatigue limit defined by crack-growth threshold for Series B specimens. In addition it was noted that the presence of a burr resulted in a decrease in fatigue life, since the burr facilitated the crack initiation process.