Fatigue Crack Initiation Behavior of Commercially Pure Titanium at Elevated Temperature

Push-pull fatigue tests were carried out on commercially pure titanium sheet at 393 K and 573 K in order to investigate the effect of elevated temperature on fatigue crack initiation behavior. The static and fatigue strengths decrease with the increase of temperature. At elevated temperatures, the sharp knee in S-N curve observed at room temperature disappears. Successive observations on specimen surface and measurements of strain hardening exponent have shown that the mechanism of crack initiation differs profoundly due to the increase in temperature. At a room temperature, fatigue microcracks initiate at a few tenth percent of fatigue life, along slip bands within not a single but multiple neighboring grains. At elevated temperatures, however, they initiate at less than 10% of their lives, along grain boundary where crowd slip bands impinge. This is ascribed to the activation of slip deformation and enhancement of strain hardening at elevated temperatures.