Global Journal of Science Frontier Research, A: Physics and Space Science, Volume 23 Issue 11

Damageability of Metals under Impulse Loading 1 Year 2023 23 Frontier Research Volume XXIII Issue ersion I VXI ( A ) Science © 2023 Global Journals Global Journal of Fig. 1: Scheme of tubular sample loading with a sliding detonation wave: 1, explosive with detonator; 2, hollow thick- walled cylindrical specimen; 3, internal cavity of the sample; 4, plugs preventing the penetration of detonation products into the sample. Fig. 2: Scheme of sample loading with the plate impact: 1, overhead explosive charge; 2, impactor plate; cylindrical sample with an axial hole. Fig. 3: Glass-shaped sample with a spalled “plate” after an end impact with a plate Impact loading using a plate accelerated by an overhead explosive charge is a well-known method, which is used, for example, for the determination of the spall strength of a material [17]. The method is convenient for preparing samples (spall “plates”) to study the microstructure of LSBs. In this case, the sample has the shape of a beaker with a thick bottom, as shown in fig. 3. The side faces of the beaker hold the emerging spall “plates” in the sample. A hollow thick-walled metal cylinder (length 70 mm, wall thickness 7 mm, outer diameter 20 mm) was used as the sample under study. The sample-beaker had a length of 20 mm, a bottom thickness of 10 mm, and a thickness of the side faces of 5 mm. The microstructure study of the samples retained after an explosive impact was carried out using a Neophot-30 optical microscope, a LEO 1450 scanning microscope, and a Zeiss Ultra Plus ultrahigh-resolution electron microscope. b) Stability loss of the converging shock wave front Figure 4 shows the cross-section of a copper hollow cylinder after pulse compression. It can be seen that the relief of the inner surface of the sample has become wavy. The presence of a surface relief of the internal cavity with folds and depressions indicates a stability loss of the shock wave: “bulging” (protrusions) appear at its front. The reason for instability of motion is the narrowing of the flow in cylindrical samples with arising a circumferential compressive stress, which induces disturbances (protrusions) at the shock wave front. II. S pallation M odel of S train L ocalization It was shown for the first time [12, 13] that strain localization is a result of high-velocity tension of the material rather than thermal softening. Under explosive loading conditions, LSBs arise in the interference zone of unloading waves, where tension stresses do not exceed the dynamic strength and the material retains its continuity. Essentially, a localized strain band is an incomplete spall crack. Spalling is a type of dynamic destruction of a material due to the interference or focusing of unloading waves where tension stresses exceed the spallation strength. Spall cracks develop into LSBs when the tension stresses in the interference zone of unloading waves becomes less than the spall strength of the material. The spall nature of LSBs formation means that the strain localization depends on the sample geometry and is practically independent of the properties of the material. The geometric factor of arrangement of the free faces, which are sources of unloading waves, is a tool that makes it possible to control the plastic strain localization process and to determine the site of of damage appearance in a sample. Under controlled conditions, localized bands of various types were obtained (planar, channel, angular, needle-like, radial, and annular [14]), which made it possible to formulate the conditions for the origin and development of LSBs, i.e., to solve the problems that the thermomechanical localization model failed to solve. a) Materials and Methods The shock load on a sample is traditionally carried out using an explosive charger. To comprehensively compress the sample, the charge surrounded the sample, being placed on its outer surface. The loading was carried out by a sliding detonation wave, which generated a shock wave in the sample. This scheme is widely used to study strain localization [15] during the collapse of cylindrical shells [16].