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Acta Metall Sin  2016, Vol. 52 Issue (5): 583-591    DOI: 10.11900/0412.1961.2015.00511
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EFFECT OF SURFACE ROUGHNESS ON VERY HIGH CYCLE FATIGUE BEHAVIOR OF Ti-6Al-4V ALLOY
Lina ZHU,Caiyan DENG(),Dongpo WANG,Shengsun HU
School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China
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Abstract  

Ti-6Al-4V alloys are widely used in aero engine blades for their unique properties, such as high specific strength, high specific stiffness and high fatigue strength. Aero engine blades usually suffer a variety of cyclic loading during the period of services, which finally results in fatigue failure. Fatigue life of materials is known to highly depend on the surface quality. Consequently, more and more researches about the influence of machined surface roughness on the fatigue behavior of materials have been carried out in the last decades. However, there are less relevant results about the relationship between surface roughness and very high cycle fatigue (VHCF) properties of Ti-6Al-4V alloy. To investigate the effects of surface roughness on fatigue properties of Ti-6Al-4V alloy under very high cycle fatigue regimes, ultrasonic fatigue tests were conducted at the conditions of 20 kHz and stress ratio R1=-1 at room temperature in air. During ultrasonic fatigue testing, each specimen was water-cooled. The specimen surfaces were cut and grinded which gave different surface roughnesses. The surface roughness was characterized using profilometry. In order to explain the high dependence of stress-fatigue life curves on the surface roughness, an approach based on the finite element analysis of measured surface topography was proposed. The results show that the VHCF property of Ti-6Al-4V alloy was significantly affected by surface roughness. The critical flaw size was 0.49~1.10 μm when the ratio between spacing and height of circumferential grooves was between 2~10. When surface roughness was smaller than the critical flaw size, surface roughness exerted no influence on fatigue life. While surface roughness was greater than critical flaw size, fatigue life decreased with increasing surface roughness. Surface roughness played a more important role in long life regime than that in VHCF regime in which with the growth of surface roughness, the crack initiation site changed from single one to two or more ones, as well as changed from inside to subsurface. When the surface roughness was large enough, all cracks initiated from surface even in super long life regime.

Key words:  Ti-6Al-4V alloy      very high cycle fatigue      surface roughness     
Received:  30 September 2015     
Fund: Supported by National Natural Science Foundation of China (No.51375331)

Cite this article: 

Lina ZHU,Caiyan DENG,Dongpo WANG,Shengsun HU. EFFECT OF SURFACE ROUGHNESS ON VERY HIGH CYCLE FATIGUE BEHAVIOR OF Ti-6Al-4V ALLOY. Acta Metall Sin, 2016, 52(5): 583-591.

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https://www.ams.org.cn/EN/10.11900/0412.1961.2015.00511     OR     https://www.ams.org.cn/EN/Y2016/V52/I5/583

Fig.1  Microstructure of Ti-6Al-4V alloy
Fig.2  Geometry and size of Ti-6Al-4V ultrasonic fatigue specimen with constant cross-section in the midst (unit: mm; R—radius, M—metric screw thread, ?—diameter)
Fig.3  Geometry and dimension of surface groove (a—width of groove, c—depth of groove, L—length of groove)
Fig.4  Finite element model of Ti-6Al-4V fatigue specimen with surface groove (σ—stress)
Fig.5  Surface topographies (a~e) and roughness profiles (a1~e1) of Ti-6Al-4V fatigue specimens for Sample 1 (a, a1), Sample 2 (b, b1), Sample 3 (c, c1), Sample 4 (d, d1) and Sample 5 (e, e1) (Straight lines in Figs.5a~e show the samplng paths)
Sample Ra / μm Ry / μm
1 0.05 0.51
2 0.15 0.68
3 0.47 1.73
4 0.65 2.97
5 7.43 18.34
Table 1  Surface roughnesses of Ti-6Al-4V alloys
Fig.6  Stress-fatigue life (σ-Nf) curves of Ti-6Al-4V alloys after very high cycle fatigue test at different surface roughnesses (Solid and half solid symbols represent fatigue crack initiation from surface and inner side, respectively)
Fig.7  Morphologies of crack initiation sites for Sample 1 after very high cycle fatigue test
Fig.8  Morphologies of crack initiation sites for Sample 3 after very high cycle fatigue test
Fig.9  Morphologies of surface crack initiation sites for Sample 4 after very high cycle fatigue test (σ =422 MPa, Nf =1.13×108 cyc)
Fig.10  Morphologies of surface crack initiation sites for Sample 5 after very high cycle fatigue test (σ =350 MPa, Nf =2.60×108 cyc)
Fig.11  Stress nephograms around groove on Ti-6Al-4V fatigue specimen with c=1.6 μm, a=16 μm and L=600 μm
Fig.12  Stress concentration factor (Ks) for the groove on Ti-6Al-4V fatigue specimen for a/c=2 (a) and a/c=10 (b)
Fig.13  Effects of surface roughness on σ-Nf curves and crack initiation position of Ti-6Al-4V alloy
Sample 2a / μm c / μm Calculated fatigue strength / MPa Modified fatigue strength / MPa Experimental fatigue strength / MPa
2 20 0.7 468.4 604.2 600
3 15 1.6 447.9 577.8 575
4 16 3.0 422.7 545.3 520
Table 2  Calculated fatigue strength and modified fatigue strength compared to experimental fatigue strength for Ti-6Al-4V alloys
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