PSI - Issue 65

Mikhail Sokovikov et al. / Procedia Structural Integrity 65 (2024) 269–274 Author name / Structural Integrity Procedia 00 (2024) 000–000

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1. Introduction

Plastic strain localization is considered as the appearance of high plastic strain gradients in relatively small (narrow) regions and is of great theoretical and practical interest. Plastic strain localization in dynamically loaded materials is known as a complex process dependent on the strain rate, temperature and the evolution of the material structure. Nowadays there are two key concepts of the occurrence of strain localization mechanisms: thermoplastic instability and structural evolution mechanisms. In ( Grady D. E. & Kipp M. E.(1987), Bai Y. L.(1982), Clifton R. J. et al. (1984), Molinari A.(1985),(1988), (1997), Molinari A. & Clifton R. (1983), Wright T. W. (1992), Wright T. & Ockendon H. (1996), Wright T. W. & Walter J. W. (1987), Zhou F. et al. (2006), Yang Y. et al. (2008)), thermoplastic instability mechanisms were taken into account, which made it possible to predict the onset of shear bands and to evaluate their thickness and the distance between them in the case of multiple localization bands. In (McDowell D. L. (2010), Austin R. A. & McDowell D. L. (2011)) , it was shown that, under dynamic loads, the behavior of the material depends on its microstructure (grain sizes, grain orientation distribution, dislocation density, dislocation substructures, etc.). In (Bronkhorst C. et al. (2006), Cerreta E. et al. (2009)), the mechanism governing the formation of plastic flow bands at high strain rates is associated with the processes taking place in the material microstructure. According to (Rittel D. et al. (2006), Rittel D. (2009), Osovski S. et al. (2012)) the dynamic failure of crystalline solids can be initiated by structural transitions (dynamic recrystallization). Analysis of the formation of localized shear bands with consideration of their spatial self-organization, growth rate and characteristic interaction time was carried out in ( Grady D. E. (1992), Nesterenko V. F. et al. (1998), Nesterenko V. F. et al. (2000), Xue Q. et al. (2002), Marchand A. & Duffy J. (1988), Giovanola J. H. (1988), Yang Y. et al. (2008), Yang Y. et al. (2011)). In this paper, we present the experimental justification for the strain localization mechanism in dynamically loaded materials induced by the collective multiscale behavior of typical mesoscopic defects (microsheаrs), which was established in Naimark O.B. (2003), (2004). The mechanisms of plastic strain localization in the material dynamically loaded on a split Hopkinson pressure bar were investigated using the specimens made of aluminum alloy AMg6, which exhibits the "tendency" to plastic flow instability. The processes of plastic strain localization were investigated in shear mode using skewed cylindrical specimens, Meyer L. W. et al. (1994). To identify the characteristic stages of strain localization and localized shear-driven failure, the thermodynamics of deformation was investigated in-situ by measuring temperature fields with a high-speed infrared camera CEDIP Silver 450M , Sokovikov M. et al. (2016). The main characteristics of the camera are as follows: sensitivity not less than 25 mK at 300 K, spectral range 3-5 μm, maximum frame size 320x240 pxl, coordinate resolution ("pixel size") ~ 0.2 mm, time resolution ~ 0.25 ms. It was shown in Bilalov D.A. et al. (2018) that, at the considered strain rates of ~10 3 s -1 and higher, the characteristic time of thermal conductivity for the AMg6 alloy significantly exceeds the time of deformation, and therefore the real-time study of temperature fields allowed one to draw conclusions about the distribution of temperature and plastic strain in the specimen. The specimens, schemes of the experiments and the obtained results are given in Fig. 1, 2. During the deformation process, temperature fields were obtained “in-situ”. Figure 1 presents the infrared image for the tested AMg6 alloy and the diagram illustrating the dependence of temperature on the coordinate at the chosen instant of time. Figure 2a gives a schematic diagram of the experiment in which the split Hopkinson pressure bar is used to impose a load on a specimen. Figure 2b presents the sketch of the skewed specimen subjected to loading. The specimen temperature field is shown for the moment when the temperature reaches its maximum during the experiment. The error of temperature measurements is ~ 10%. 2. Materials and Experimental Methods