PSI - Issue 17

Tatyana Tretyakova et al. / Procedia Structural Integrity 17 (2019) 906–913 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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decrease of strength, plasticity and reduce quality of material’s surface as well. Bending and the spontaneous macroscopic localization of plastic flow lead to a non-uniform thickness, and, as a consequence, to macroscopic fracture. Therefore, it is important to obtain experimental data on the behavior of structural materials under complex stress conditions taking into account the spatial-time inhomogeneity of plastic flow due to the Chernov-Lüders deformation and the Portevin-Le Chatelier effect [Yin et al (2014), Aguirre et al (2004), Daghfas et al (2017), Zhang et al (2005)]. Mechanical tests under complex stress conditions require, as usual, the use of specialized equipment and devices, which cause methodological difficulties in the procedure of experiments. It is noted that a significant drawback of the existing methods of experimental study of the mechanical properties under complex stress is the difficulty of testing set up with using expensive equipment, maintenance difficulties due to the presence of pumps, high-pressure multipliers, and systems for additional loading of specimens. The substantial development of optical methods of experimental mechanics allows us to register non-uniform processes of plastic deformation, accumulation of defects and the development of cracks in a material in a contactless way. This work proposes the use of specimens with specialized complicated geometry, in the working part of which a complex stress state is achieved during uniaxial loading. In this paper, we propose test methods using thin-walled tubular specimens with joint tension with torsion loading, as well as specimens with complicated geometry. In particular, special focus will be on the samples in the form of plates located inside a rigid circular rim and rim of reverse curvature. Under uniaxial tension of these samples, a flat stress state occurs with the main stresses of different signs in the plate. In order to assess the influence of the type of stress state on the regularities of the spatial-time heterogeneity of plastic flow of structural metals and alloys, mechanical tests for uniaxial tension of experimental samples in the form of plates with a rigid rim of two types were carried out. The geometric parameters are determined as a result of numerical modeling. The first type is the plate with a rigid circular rim, in the working part of which tensile forces are realized along the axis Oy (along the loading axis) and compressive forces in the transverse direction (along the axis Ox) due to the curvature of the rim and its narrowing in the transverse direction. The second type is the plate with a rigid rim of inverse curvature (in the shape of an hourglass), the geometry of which occurs biaxial tension in the working part under uniaxial loading of the sample. Specimens are made by milling on a machine with numerical control (CNC) from the sheet Al-Mg alloy (with technological plating, GOST 17232-99) with a thick of 12 mm. The chemical composition of Al-Mg alloy: Mg – 6.1 %, Mn – 0.6 %, Fe – 0.2 %, Si – 0.1 %, Cu – 0.1 %, Zn – 0.1 %. The material was tested in the delivery state without additional heat treatment. The loading was carried out on the Instron 5989 universal electromechanical testing system (600 kN) with a constant displacement speed of 5 mm per minute (Fig. 1). Study of the macroscopic localization of plastic deformation due to the jerky flow is based on analysis of inhomogeneous deformation and temperatures fields. The evolution of inhomogeneous strain fields was registered by using the 3D-digital-image-correlation measurement system Vic-3D. The images were captured with a pair of two CCD black/white cameras (Prosilica, 16 Mp resolution) at a frequency of 3 Hz. Fields of longitudinal, transverse and shear strain were calculated by using the Lagrange tensor with the Oy axis directed along the tension axis and the Ox axis was perpendicular to it. The temperature fields are measured by the infrared thermography, that is, the infrared camera Flir SC7700 with a resolution of 640×512 pixels was applied. It is important to notice that the software and hardware synchronization of the testing system and measurement equipment was carried out with the DAQ device NI USB-6251. Both surfaces of specimens were prepared in order to improve accuracy. The preparation of surface from the side of the Vic-3D system included spraying the black/white speckle pattern (set of random black dots on the white matt base). From the side of the IR-camera the surface was covered with a black matt coat to reduce reflections from the surface. 2. Experimental study of the jerky flow 2.1. Specimen geometry and testing procedure