Effect of Stress Ratio(R) and Stress Concentration Factor (Kt) on Fatigue Properties of WSTi6211 Titanium Alloy

In this paper, the author studied the effects of different stress ratios(R) and stress concentration factors(Kt) on the fatigue properties of WSTi6211 titanium alloy.Through S-N curve, the author obtained the fatigue ultimate strength of the material under different conditions and analyzed characteristics of fatigue fractures, including the crack source, the crack growth region and the final rupture region. The results show that when Kt=1, R=0.5, the fatigue ultimate strength σD is 626MPa; when Kt=1, R=0.06, the fatigue ultimate strength σD is 527.5MPa; when Kt=3, R=0.06, the ultimate fatigue strength σD is 267MPa. Fatigue performance is very sensitive to R and Kt. The larger R is, the larger the fatigue ultimate strength is. The larger Kt is, the smaller the fatigue limit strength is. The fracture morphology shows typical fatigue fracture morphology. Most of the cracks originate on the surface of specimens and have typical fatigue bands. With the decrease of stress, the area of crack growth zone increases.


1.Intrpduction
"A generation of materials, a generation of equipment". [1] Titanium alloy is an advanced aviation material with a density of only 4.5g/cm 3 . It has excellent corrosion resistance, higher specific strength, good high temperature performance and fatigue resistance, and has become one of the main structural materials of modern aircraft. At present, the use of titanium alloys in aircraft has become one of the important indicators to measure its advancement. For example, the use of titanium alloys in F-22 of the fourth generation aircraft in the United States has reached 41%. With the development of aviation technology, the requirement of combat performance, maneuverability, reliability and service life of aircraft has been improved, and the amount of high-end titanium alloys used in new aircraft will continue to increase.
WSTi6211, similar to TA15 titanium alloy, is a near-alpha titanium alloy with high [Al]eq. It has medium room temperature and high temperature strength, good thermal stability and weldability. It is an important titanium alloy material for aircraft and can be used for a long time from 450℃ to 500℃ [2] . Fatigue property of materials is usurally an important factor that directly affects their service life. There are many reports on the study of microstructure, fatigue properties and fracture behavior of titanium alloys. However, most literatures focus on the comparison of fatigue deformation characteristics between lamellar structure obtained by single-phase deformation and bimodal structure obtained by two-phase deformation [3][4][5][6][7][8] . However, the effect of R and Kt on the equiaxed structure of the alloy has rarely been reported. In this paper, the effects of Kt and R on fatigue properties were studied, and the fracture morphology characteristics were analyzed, which lays a theoretical foundation for exploiting the potential application of materials. The microstructure of WSTi6211 titanium alloy bar is typical equiaxed structures processed in two-phase zone.

Fracture Morphology Observation
After the fatigue loading test, the fracture morphology of specimens under different loading conditions was observed by JSM4640 SEM. The characteristics of fatigue crack source, propagation zone and instantaneous fracture zone from high stress zone to low stress zone were observed. The sensitivity of the alloy to stress ratio and stress concentration factor was analyzed.

Fatigue Performance of Kt=1,R=0.5
Under the test conditions of Kt=1 and R=0.5, the median fatigue life corresponding to the maximum stresses of 1000 MPa, 900 MPa and 800 MPa is 6.8×10 4 、 2.58×10 5 and 1.108×10 6 respectively. The fatigue ultimate strength was measured by lifting method. As shown in Table 2

Effect of R on Fatigue Properties
Comparing

Effect of Kt on Fatigue Properties
Comparing

Conclusion
(1) R has a great influence on the fatigue properties of WSTi6211 titanium alloy. With the increase of R, the_D of the material increases. When R increases from 0.06 to 0.5, the corresponding_D increases from 527.5 MPa to 626 MPa.
(2) Kt has a great influence on fatigue properties of WSTi6211 titanium alloy. When Kt increases from 1 to 3, the corresponding_D decreases from 527.5 MPa to about 50%.
(3) The fracture morphology shows typical fatigue fracture morphology. The crack initiation point is more than the sample surface, and there are typical fatigue bands. With the decrease of stress, the area of crack propagation area increases.

Acknowledgements
Thank my tuter, classmates and colleagues and they gave me lots help during experiments.