PSI - Issue 50

Mikhail Nadezhkin et al. / Procedia Structural Integrity 50 (2023) 206–211 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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local corrosion and sudden failure. The nature of spontaneous plastic strain localization at the macrolevel, the physical mechanisms of spatially correlated propagation of deformation bands, and especially their role in ductile fracture have not yet been fully understood. Up to now, these questions have been discussed in terms of the temperature and strain rate dependence of the jerky flow characteristics and deformation band properties. There are at least the following reasons for the increasing interest in the problems of intermittent plasticity and deformation banding in the last decade. The first reason is the development of dynamic analysis and its penetration into various fields of the natural sciences, including materials science. In this context, the Poretevin – Le Chatelier (PLC) effect is considered as a representative nonlinear phenomenon associated with the spontaneous formation of spatio-temporal (dissipative) structures in the form of propagating deformation bands. The second reason is the increase in the world production of rolled aluminum alloy products (prone to jerky flow) for automotive and aerospace industries. The last reason is the development of laser welding technology used in the production of aircrafts and space vehicles. A classification of PLC deformation bands was proposed by Shibkov et al. (2016) based on the data of high-speed video recording of the surface of flat Al – 5% Mg alloy samples deformed at a constant rate. Bands of type A propagate continuously, bands of type B hop discontinuously, and bands of type C nucleate at random points on metal surface and do not propagate. Despite numerous experimental (e.g. Kocks (1976), Yavari et al. (1981),Wilshire and Burt (2003), Zhan et al. (2011), Shibkov et al. (2016), Yuzbekova et al. (2017)) and theoretical (Chirkov (2021)) studies of band formation kinetics in recent decades, there remain many questions unanswered. The most important and still unresolved issues concerning plastic strain localization are as follows: (1) the nucleation mechanisms of deformation bands, (2) the spatial correlation mechanisms underlying the band propagation, and (3) the role of spatio-temporal structures of deformation bands in the necking and fracture of the material. Some progress in determining the zones of localized creep deformation has been made with the use of ultrasonic nondestructive testing. (e.g. Valluri et al. (2010) and Hall et al. (2014)) However, these methods require that the analyzed surface must be equipped with arrays of sensors, which is sometimes difficult to implement. Deformation behavior in polycrystalline aluminum under high-rate loading was observed by the digital image correlation method by Mello (2016). Barannikova et al. (2004) and Nadezhkin and Barannikova (2021) found that macroscopic plastic deformation under loading at a constant rate (active deformation) is localized starting from the yield point to fracture, and the localization pattern depends on the stages of plastic flow. However, creep testing is often more informative, as it allows one to determine the type and activation parameters of micromechanisms governing plastic flow (e.g. Wilshire and Burt (2003)). This paper discusses the features of plastic strain localization in commercially pure aluminum during creep at room temperature at different stress levels above the yield point. 2. Materials and methods The investigation was performed on flat dog-bone specimens of commercially pure 1050A aluminum (aluminum content of at least 99.5%) with the gauge dimensions 50×10×2 mm. The specimens were stamped from sheets after preliminary 10% tensile deformation and recrystallization annealing at 400 0 С for 1 hour. The grain size in the specimens after such treatment varied within 2±1 mm. Creep tests were carried out at 300 K on a Walter + Bai AG LFM-125 universal testing machine with a frequency of load synchronization of 5 Hz and a the maximum speed of maintaining the load of 300 N/s. The range of creep stresses varied within 51 – 58.5 MPa; during this interval the total elongation strain of the specimen was continuously recorded as a function of time ε(t) . Mechanical tests were performed in combination with strain field analysis using two digital methods: digital image correlation (DIC) and digital statistical speckle photography (DSSP). The latter has an order of magnitude greater temporal and spatial resolution than DIC (Pan et al. (2009), Zuev et al. (2010)). This allowed us to observe in situ the localized plasticity pattern, which in this case is the main characteristic of deformation processes in the material, and to obtain quantitative information about the distribution of the plastic distortion tensor components over the deformed specimen and their temporal evolution. The analysis was performed based on chronograms of propagation of deformation fronts X(t) (time dependences of the position of the central point of the deformation front). They contain quantitative information about the magnitudes of the front velocities and the front propagation modes. The experiment was conducted using the ALMEC-tv measuring complex that operates in DIC and DSSP