Abstract
Torsion fatigue tests were conducted for smooth bars and circumferentially notched bars of Ti-6Al-4V alloy. The purpose of this work is to evaluate fatigue strength and notch sensitivity under stress concentration factors of Kt=2.0 and 4.0. Fatigue life and fatigue strength decrease with increasing stress concentration. To investigate the crack initiation and propagation, the crack length were calculated with Hooke's law. When compared at the same fatigue life, the crack initiation life for Kt=4.0 was much shorter than that for Kt=2.0. Difference of the fatigue fracture mechanisms between blunt and sharp notch is considered to cause this phenomenon. Under low stress amplitude, the crack propagates under tensile mode toward a 45° to the axial direction. The crack propagated all over the blunt notch, and the crack propagated in the cross section of the sharp notch root under low stress amplitude. As a result, the fracture surface of blunt notch specimen under low stress amplitude was flat, and that of sharp notch specimen showed factory-roof pattern. The formation of factory-roof increases contact areas in the fractured surface and lengthens crack propagation time.
