PSI - Issue 28

S.A. Atroshenko et al. / Procedia Structural Integrity 28 (2020) 101–105 Author name / Structural Integrity Procedia 00 (2019) 000–000

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In the second zone, the transformation process is caused by activation of the plastic flow and local heat release and is characterized by localization of shears in the form of adiabatic shear bands (fig. 3a), tear-off microcracks up to 150 μm in length (fig. 3b) and regions of dynamic recrystallization (fig. 3c).

a

b

c

Fig. 3. Dissipation of the energy of the projectile in the target alloy 3M by localizing the plastic flow of rotational deformation modes - (a); the formation of microcracks - (b); recrystallization - (c).

In the third zone, where, in addition to the deformation processes associated with the penetration of the projectile, processes occur due to the reflection of the shock wave, which is caused by the collision of the projectile and the target and at the same time far ahead of the projectile, from the free surface. Due to the interference of the direct and reflected waves, dynamic tensile stresses arise, which destroy individual sections of the third zone. Thus, adiabatic shear bands are visible in different parts of this zone, which, due to reflection effects, pass into cracks and, as a result, separate target fragments appear in this zone (fig. 4a - 4c). Note that fragment formation begins as a structural-phase process of discontinuity during the self-organization of two bands of localized plastic deformation. Structural changes in the third zone of the target weakly depend on the initial metal structure and are mainly realized with the participation of interference (wave) effects of scattering of the shock wave generated during the interaction of the projectile with the obstacle.

Fig. 4. Fragmentation of the target and the formation of adiabatic shear bands: (a), (b) – adiabatic shear bands turning into cracks; (c) adiabatic shear bands parallel to the shores of the cavity — prototypes of microcracks.

Measurement of microhardness along the coast of the cavity from top to bottom showed that the smallest values of microhardness (HV = 2.25 - 2.35 GPa) are characteristic of sites located near the impact surface, in the middle of the target thickness – HV = 2.46–2.53 GPa, at the end of the third zone – HV = 2.88– 2.93GPa. That is, it can be assumed that the change in microhardness is associated with a change in the strain intensity: at the collision surface, the deformation is minimal, respectively, less microhardness, in the third zone, the deformation is maximum - the microhardness is much higher.