Fatigue behavior in notched component of (alpha+beta) and beta titanium alloys under combined axial-torsional loading

In the present study, fatigue tests under axial, torsional and combined axial-torsional loading were conducted using the cylindrical specimen with circumferential blunt notch of Ti-6Al-4V titanium alloys. Two kinds of alloys with different microstructure, the (alpha+beta) and the beta alloys, were investigated in the fatigue tests. The fatigue life was correlated with the range of equivalent plastic strain. In both alloys, the fatigue life for the same equivalent plastic strain became longer as increasing shear component in the stress state at the notch root. The dominant surface cracks propagated in Mode I under the axial and combined loading in the two alloys. When the crack growth rate under the axial and combined loading was correlated with the range of J-integral, the result almost coincided with the growth relation for large through-thickness cracks in each material. Although the growth of Mode ii type was predominant under the torsional loading, the growth direction of the main crack was the axial one in the (alpha+beta) alloy, but the circumferential one in the beta alloy. The cracking morphology depending on the microstructure, under the torsional loading, was simulated successfully by using a proposed model for crack initiation.