PSI - Issue 41

Aleksandr Inozemtsev et al. / Procedia Structural Integrity 41 (2022) 510–517 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Fig. 2. (a) A specimen VT6 placed between the two bars of the split Hopkinson pressure bar for pre-dynamic loading (for clarity, the sleeve is removed); (b) the original and tested samples for dynamic stretching. 3. Results and discussion Fatigue tests were carried out using the resonance type of Shimadzu Ultrasonic Fatigue Testing Machine USF 2000. It has the advantage of being able to provide cyclic loading (R= – 1) through the use of a generator, which transforms the frequency of 50 Hz into ultrasonic electrical sinusoidal signal of frequency 20 kHz by means of piezoelectric transducer, generating longitudinal ultrasonic waves at a frequency 20 kHz, as well as to produce the mechanical stresses with maximum amplitude in the gauge length (mid-section) of the specimen. The specimens were cooled with compressed air. The geometry of the specimens is given in Fig.3. The frequency deviation of 0.5 kHz was associated with the damage critical stage and considered as a failure precursor related to the initiation of a crack with characteristic size of ~2 mm. The level of applied stresses allowed us to investigate fatigue strength of specimens tested at stress amplitudes of 130 - 470 MPa in the HCF regime up to 10 10 cycles until failure.

Fig. 3. Geometry of the specimen used in cyclic tests in the gigacycle fatigue regime, the dimensions are in mm.

It has been found that the fatigue stress of preloaded VT6 titanium alloy decreases from the stress level of 460 MPa to 130 MPa, which corresponds to the critical number of cycles ~  10 8 (Fig. 4). The experimental study of fatigue failure in the transient High Cycle and Very High Cycle fatigue regimes (number of cycle N~10 6 -10 8 ) revealed a regular decrease of the fatigue stress in dynamically preloaded specimens and the existence of the fatigue limit of ~130 MPa.