PSI - Issue 18

Tatyana Tretyakova et al. / Procedia Structural Integrity 18 (2019) 837–842 Author name / Structural Integrity Procedia 00 (2019) 000–000

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High reliability of created structures and prevention of industrial accidents and disasters are largely determined by the availability of information about physical, mechanical and strength characteristics of materials used, considering the influence of real mechanical and heat external actions determining material’s complex stressed state. Distraction of structural elements is caused by material’s elastoplastic deformation associated with alteration of its structure and physical and mechanical properties during loading. Effects of serrated yielding lead to a significant decrease of strength and plasticity, reduce material surface quality. Bending and the spontaneous macroscopic localization of plastic flow lead to a non-uniform thickness, and, as a consequence, to macrofracture [Yilmaz (2011), Aguirre et al (2004), Daghfas et al (2017), Zhang and Jiang (2005)]. It is of interest to analyze the mechanisms and patterns of plastic deformation, to establish the relationship between the phenomena of spatial inhomogeneity and the time instability of plastic deformation with processes taking place at various scale levels under complicated loading. It seems promising to obtain and systemize the experimental data on principles and features of serrated yielding under complex mechanical effects based on analyzing the kinetics of strain and temperature fields, acoustic emission signals during initiation and propagation of macroscopic localization due to the jerky flow by the example of structural steels and Al-Mg alloys. In particular, the influence of additional vibration on the Chernov-Lűders and the Portevin-Le Chatelier effects is considered. 2. Experimental study 2.1. Material and specimen geometry The materials studied in this investigation is the aluminum-magnesium alloy (the chemical composition: Mg – 2.2 %, Mn – 0.6 %, Fe – 0.4 %, Si – 0.4 %) and the structural carbon steel (the chemical composition: Fe – 99 %, C – 0.18 %, Si – 0.20 %, Mn – 0.35 %, Cr – 0.04 %, Ni – 0.03 %, Cu – 0.04 %). Plane dog-bone specimens were cut by the hydro-abrasive treatment from rolled sheets of Al-Mg alloy and steel 20, thickness of 3.0 mm. Materials were studied in the state as received without additional temperature processing. 2.2. Experimental procedure The specimens were tested in tension with a biaxial (tension-compression/torsion) servo-hydraulic testing system Instron 8850 at room temperature and at the rate of 2-5 mm per minute. The research program includes mechanical tests on uniaxial tension tests and tensile tests with additional vibration impact. The vibration impact was carried out with the tension load and at the frequency of 4-8 Hz with a sinusoidal waveform. 2.3. Analysis of displacement and strain fields in tests with vibration impact In this paper, the registration of inhomogeneous displacements and strain fields was carried out by the contactless three-dimensional digital image correlation system Vic-3D. It is necessary to ensure the synchronization of test and measuring equipment, and to provide necessary conditions for camera shooting (uniform illumination of the specimen surface, no “blurred” image at vibration impact). To synchronize recording and loading processes the NI USB-6251 analog to digital converter (National Instruments) was used. Additionally, in the “WaveMatrix” software of the Instron 8850 testing system, the loading program is supplemented with brief stops at the top of the cycle (at the maximum load level), while a 1-volt signal is sent to the analog channel of the testing system controller every n-cycles. This signal is a trigger for the video system, which allows shooting only at the maximum loaded state of the sample. Recording in the same phase of the cycle provide accurate estimation of initiation and propagation processes of deformation bands, developing of defective structures and macroscopic cracks in the material. In the ‘Vic-Snap’ software of the video system recording is performed in the ‘Analog Capture’ mode, specimens were captured when the voltage level on the synchronization unit channel reach 0.9 volts (Fig. 1, b). Also it is possible to add ‘Timer’ to the recording mode for additional registration of deformation fields during loading, for example, every 60 seconds