PSI - Issue 39

Ilia Nikitin et al. / Procedia Structural Integrity 39 (2022) 599–607 Author name / StructuralIntegrity Procedia 00 (2019) 000–000

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3. Experimental procedure and results The VHCF test under pure torsion were carried out by using piezoelectric fatigue testing system [8] in laboratory air environment. The system consists of direct torsion converter, generator, computer and ultrasonic torsion horn, fig.2. The fully reversed (R=-1) fatigue tests were performed in the gigacycle region with the run-out limit of 5x10 9 cycles. The two materials were investigated: forged and extruded titanium alloys VT3-1 [9] with the next principle alloying elements: 5.5-7 Al; 2-3 Mo, 0.8-2 Cr, 0.2-0.7 Fe.

Fig. 2. The VHCF pure torsion testing system with direct piezoelectric converter [8].

The SN-curves for both materials are presented on fig.3. To compare the torsion test results with axial-ones [10] the equivalent stress was calculated based on tension-compression results. In the case of forged Ti-alloy the results are close, however the slop of torsion tests is higher. In the case of extruded Ti-alloy the results of torsion tests are above the equivalent representation of tension-compression, but the slop of torsion SN-curve is still higher.

Fig. 3. SN-curves for forged (left) and extruded (right) titanium alloys VT3-1.

The analysis of the lateral surface of the specimens, fig.4, shows the two main steps of crack path: (1) initial stage in the plane of maximum shear stress and (2) developed crack in the plane of maximum normal stresses. The orientation of the early crack growth can be as along the specimen’s axis (0 degree), as well across it (90 degree). Also, sometimes