PSI - Issue 18

Mikhail Sokovikov et al. / Procedia Structural Integrity 18 (2019) 262–267 Author name / Structural Integrity Procedia 00 (2019) 000–000

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specimen were carried out after impact tests using a FEI PHENOM G2 ProX scanning electron microscope at accelerating voltage of 15 kV and at×6000 magnification, Fig.3. Electron microscopy revealed the regions of localization of plastic strain, which as a rule were located as close as possible to the rear surface of the specimen (target) and approach to the surfaces of fracture. The structure of the localized shear was observed at a depth of the order of 80-100 μm from the fracture surface. Both, single crystals (are shown by arrows in Fig.3a) and packets of crystals (are shown by arrows in Fig.3b) were detected, which is indicative of significant evolutionary processes in defective subsystems during dynamic deformation of the test specimen. It should also be noted that the localized shear bands show the evidence for fragmentation (are shown by arrows in Fig.3c) in the regions closest to the media interface (specimen material /fracture surface), which also directly points to the processes of cold dynamic polygonization, which is an integral part of the evolution of the structure during dynamic deformation Rittel D. at al. (2008). Microstructural studies of the morphology of fracture surfaces after ejection of the plug from the perforated target, which were based on the analysis of three-dimensional profilometering (New View) data of the specimens made from the aluminum alloy AlMg6, allowed us to obtain the distribution of the scale-invariant index (Hurst index), Oborin V.A. at al (2015) along the fracture surface (Fig. 4). The Hurst exponent was defined from the slope of linear portion of the correlation function in logarithmic coordinates. The spatial range of linear part establishes the correlated behavior of defect induced roughness in the direction of projectile propagation. Two fracture regions were detected along the generatrix of the plug: a homogeneous rough surface, corresponding to the localization of plastic strain, and coarse fracture surface, corresponding to brittle fracture and adjacent to the rear surface. A transition from the zone of localization of plastic strain to the fracture zone is characterized by a sharp increase in the Hurst index.

Fig.4. The fracture surface of the specimen 10 mm thick (a) and the Hurst index in different regions (b).

Conclusions The experimental study (combined with infrared measurements and numerical modeling, Bilalov D.A.at al. (2018)) of strain localization under dynamic loading allowed us to consider a transition to fracture due to localized shear as a mechanism induced by structural changes in the material. Temperature measurements made in the localization zone did not support the traditional understanding of deformation localization mechanisms as autocatalytic temperature control of viscoplastic phenomena. The samples saved after the end of the experiment were subjected to microstructural analysis using scanning electron microscopy techniques. The analysis revealed significant evolutionary processes in defective subsystems during the dynamic deformation of test samples. Based on the obtained results, a transition to fracture due to localized shear can be treated as the process caused by structural changes in the defective subsystem of the material.