PSI - Issue 28

V.V. Balandin et al. / Procedia Structural Integrity 28 (2020) 1802–1807 / Structural Integrity Procedia 00 (2020) 000–000 V.V. Balandin et al.

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Fig. 4. (a) Dependence of the relative elongation on the strain rate. (b) Dependence of the relative constriction on the strain rate.

3. Spall Strength Experiments

Experiments on the study of the spall strength of the 3M alloy were carried out on a setup consisting of a load ing device - a gas gun with a caliber of 57 mm (Balandin et al. (2016)) and a VISAR interferometer (Barker and Hollenbach (1972)) for recording the velocity of the free surface of the sample (Fig. 5). To create plane loading waves, the samples under study were loaded by the impact of the plate. The striker is a thin-walled glass made of steel or aluminum alloy, with a titanium or steel bottom of the required thickness. The striker’s speed is determined using two electrocontact sensors located in the bore holes. The signals from the sensors are recorded by an oscilloscope. A triggering contact is located on the surface of the sample, which closes upon impact on the plate under study. The collision occurs in a vacuum chamber, to which the gun barrel is attached and into which the test sample is inserted. Circular plates 92 mm in diameter and 6 mm thick were used as samples. The striker plate thickness was 3 mm. When the compression wave emerges on the back surface of the sample, it is reflected by the tension wave. The interaction of compression and tension waves can lead to fracture and spalling in a certain section of the sample. The motion of the free surface of the sample is recorded by the VISAR interferometer. A laser with a radiation wavelength of 0.53 µ m is used as a radiation source. Probing (from the laser to the target surface) and reflected (from the target surface to the optical system) radiation are transmitted through the air. Fig. 5 uses following denotations: 1-gas gun, 2 - two-diaphragm shutter, 3 - striker (thin-walled cylinder with the bottom of the required thickness), 4 - electrical contacts for measuring speed, 5 - vacuum chamber, 6 - barrel, 7 - starting contact, 8 - sample, 9 - focusing lens, 10,13,16,20 - mirrors, 11,17 - quarter-wave plates, 12,18 - beam splitters, 14 - translucent mirror, 15 - optical delay line, 19 - photodetectors, 21 - laser. Free surface velocity profiles were measured using a VISAR interferometer (Fig. 6). On the leading edge of some velocity profiles, there are small steps in the velocity range of 150-200 m / s, corresponding to an elastic precursor. This is followed by the main compression wave with an amplitude W 1 , after which a minimum of the W 2 velocity is observed, followed by the so-called spall pulse. The presence of this impulse is associated with the reflection of a part of the extension wave from the spall section. The obtained dependences of the free surface velocity were used to determine the maximum free surface velocity W 1 , the minimum free surface velocity W 2 before the onset of the spall pulse, and the decay time of the free surface velocity from W 1 to W 2 . The maximum tensile stresses and strain rate were determined by the formulas:

1 2 ρ 0 C 0 ( W 1 − W 2 )

(3)

σ t =

1 2 C 0

∆ W 1 ∆ t

˙ ε =

(4)