Effects of Stress Ratio on Initiation and Initial Propagation of Interior-originating Crack in Ti-6Al-4V Alloy

Uniaxial fatigue tests of (α+β) Ti-6Al-4V alloy were carried out at various stress ratios (R=σmin/σmax) up to very high cycle region. The effect of stress ratio on very high cycle fatigue behavior was investigated based on SEM fractography. The S-N curves under positive stress ratios showed a complex folding shape with a steep incline in a long life region where interior-originating fractures occurred. On the other hand, the S-N curves under negative stress ratios had a general feature ; a monotonic increase of the fatigue lives with decreasing stress. SEM observations revealed that an interior crack was formed by linking of small cracks propagated from many interior origins located closely. This type of interior crack was particularly observed in low stress under positive stress ratios. The positive stress ratios facilitated the initiations of cracks and subsequent linkage of the cracks. This tendency may relate with the steep inclination of S-N curves under positive stress ratios. A clear difference was observed on the fracture surfaces between surface-and interior-originating fractures. A few micro-meter size granular substances existed only on the fracture surfaces of interior-originating fractures. The formation of the granular region was accelerated by negative stress ratios. Compressive loads may play an important role in the phenomenon.