Effect of surface integrity on bending and torsional fatigue life of titanium alloys

Abstract:

To improve the fatigue life of a vehicle bearing shaft under bending-torsion loading a microscopic finite element model containing surface roughness and surface residual stress was established for the bearing shaft material TC11 titanium alloy. A bending-torsion fatigue life prediction model of TC11 was established based on the critical plane method to study the effects of surface roughness and axial/circumferential residual stress on the fatigue life. The fatigue samples were processed by turning and ultrasonic rolling respectively, and the surface integrity of the samples was compared under both processes. Fatigue tests were carried out to verify the accuracy of the prediction model, and the fracture mechanism was studied by analyzing the fracture morphology. The results show that the influence of surface roughness and axial residual compressive stress on life is significant. As the surface roughness S_a decreases from 1.6 μm to 0.4 μm, the life is increased by 135%, and the axial residual compressive stress increases from 100 MPa to 400 MPa, the life is increased by 123%, and the influence of circumferential residual stress is small. The rolling specimens have lower surface roughness, higher work-hardening degree, higher axial/circumferential residual compressive stress, and higher fatigue life. The SWT model can accurately predict the bending and torsional life of TC11 within the 25% error scatter band, which provides a theoretical basis for process optimization. After rolling, the fatigue grain spacing decreases from 0.8-0.9 μm to 0.3-0.4 μm, the crack propagation rate slows down, and the main fracture mode transitions from intergranular fracture to transgranular fracture in the machining-affected layer.