PSI - Issue 41

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

512

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2. Materials and experimental conditions Fatigue tests were conducted in two steps. First, specimens made of VT6 ( Тi - 6Аl -4V) titanium alloy were subject to dynamic preloading on the split Hopkinson pressure bar at the strain rates of ~10 3 s -1 , and then tested in the cyclic loading regime at room temperature. The chemical composition (in weight %) of the alloy is shown in Table 1 and its tensile properties are summarized in Table 2.

Table 1. Chemical composition of VT6 grade (in weight %). Al V Zr Si Fe C

O

N

H

Impurity

4.51

4.38

0.04

0.02

0.12

0.007

0.136

0.006

0.003

0.085

Table 2. Quasi-static tensile characteristics of VT6 grade. Elastic modulus (GPa) Yield stress (MPa)

Ultimate elongation (%)

Tensile strength (MPa)

115

814

950

16

In our experiment the split Hopkinson pressure bar (SHPB) technique was used for pre-loading of specimens under conditions of high-rate tension. The SHPB consists of a gas gun and three cylindrical bars (Fig. 1), known as the striker bar (SB), incident bar (IB) and transmission bar (TB). The gas gun is used to propel the striker bar towards the far end of the incident bar, which is some distance away from the specimen. The impact between the two bars generates a compression pulse, which freely passes through the holder and the specimen to the transmission bar without causing plastic deformation in the specimen, because the main part of the wave propagates through the holder showing higher yield limit.

Fig. 1. Split Hopkinson pressure bar experimental apparatus.

Upon reaching the free end of the transmission bar the compression wave is reflected as a tensile wave. This tensile pulse is the initial incident wave that causes stretching of the specimen. As soon as the tensile pulse reaches the specimen, it splits into two waves, one of which travels through the specimen into the transmitted bar, causing plastic deformation in the specimen. The other wave is reflected away from the specimen and travels back through the incident bar. Note that in this case the specimen undergoes plastic deformation in the region adjacent to the smallest cross section, while the holder being disconnected with the bars experiences no tension (Fig. 2).