Issue 35

A. Nikitin et alii, Frattura ed Integrità Strutturale, 35 (2016) 213-222; DOI: 10.3221/IGF-ESIS.35.25 217 3     . It can be pointed out that points plotted in equivalent stress terms for forged VT3-1 show a good agreement and grouped near to 400 MPa that is about 40 % of UTS. This result is similar to typical results on titanium alloys in the HCF range. (a) (b) Figure 4 : Results of fatigue tests on forged VT3-1 titanium alloy (a) torsion data in terms of shear stress amplitude and (b) tension compression R=-1 and torsion data in terms of equivalent normal stress amplitude. The torsion results cover a range of fatigue life from about 10 6 to 10 8 cycles. Based on previous results obtained on forged VT3-1 under fully reversed tension loading, the crack initiation mechanisms could be quite different at such fatigue life for forged alloy [16]. Besides a changing from surface to subsurface crack initiation mode, it has been reported about micro- structural fatigue life controlling mechanisms. In the case of torsion loading, a transition from surface to subsurface initiation after longer fatigue life was also found. However, this transition appears at fatigue life of about two order of magnitude longer. Detailed analysis on fatigue crack initiation mechanisms under torsion loading will be provided in the section ‘Discussion’. (a) (b) Figure 5 : Results of fatigue tests on extruded VT3-1 titanium alloy (a) torsion data in terms of shear stress amplitude and (b) tension compression R=-1 and torsion data in terms of Von Mises equivalent stress amplitude. Like for the forged alloy, the SN curve in torsion for the extruded VT3-1 titanium alloy exhibits a significant slope in VHCF regime, Fig.5a. Since the fatigue tests were stopped around 10 9 cycles, most of the cracks were observed in the fatigue life range below 10 9 cycles. As already was done for the forged alloy, the results of torsion tests on extruded VT3-1