Issue 27

T.V. Tretiakova et alii, Frattura ed Integrità Strutturale, 27 (2014) 83-97; DOI: 10.3221/IGF-ESIS.27.10 84 manifests itself as local thinning of the specimen’s transverse [13]. Analysis of fundamental and current scientific literature has revealed the relevance of the issue despite its long history [14]. Furthermore, appearance of the advanced test equipment, high-effect measuring systems and high-accuracy facilities for carrying out basic and applied research caused a great increase of scientists’ concern focused on the aspects of the macroscopic localization of plastic flow, especially the local strain bands propagation, the influence of strain rate and temperature regimes, the chemical composition, specimen geometry, grain size and orientation, etc. on the occurrence of the PLC behavior [15–19]. In this work a technique based on the digital image correlation (DIC) method has been used for study of spatial-time inhomogeneity of serrated plastic flow Al-Mg alloy. The DIC is a highly effective non-contact computer-vision-based technique, which provides estimation of the displacement and strain fields on specimen surface by matching the reference subsets in the undeformed image (before loading) with the target subsets in the deformed images (captured during test) [20]. This paper presents, in the first part, the brief description of the test procedure of carrying out experimental investigations by the combined use of a servo-hydraulic biaxial test system Instron 8850 and a non-contact 3-D digital image correlation measurement system Vic-3D [21]. The Vic-3D system can be used for problem solving of deformable solid mechanics: experimental investigation of non-uniform strain fields and analysis of failure conditions in bodies with concentrators of different geometry, research of inelastic material deformation processes in complex strain-stress conditions, study of displacement and strain fields evolution during crack initiation, damage accumulation and material failure, etс. In the following, the representative load ( P , kN) versus displacement ( u , mm) curve observed during tensile tests on an Al-Mg alloy sheet specimens is shown. Then, a detailed description of the Lüders bands, the PLC bands initiation and propagation, the correspondence between the deformation bands and the serrations on the P – u curves is given. The macroscopic localization of axial strain due to the necking effect at the post-critical deformation stage is illustrated as well. In conclusion, the recurrence in the strain distribution leveling along the specimen gauge is shown. The change between the macroscopic localization of the plastic flow, namely the running of the Lüders and PLC bands and the recovery of strain field homogeneity, has been observed. E XPERIMENTAL PROCEDURE he material used for experimental investigations of the spatial-time inhomogeneities and effects of localized plastic strain bands’ propagation is an aluminum-magnesium alloy (GOST 4784-97; 2.2% Mg, 0.6% Mn, 0.4% Fe, 0.4%Si). The research program included tests on uniaxial tension of the flat dog-bone tensile specimens (Fig. 1) with the geometrical parameters shown in Tab. 1. The samples were made in accordance with the Russian Standard GOST 1497-84 « Metals. Test Methods on Tension ». Figure 1 : The sketch of the flat dog-bone tensile specimens (GOST 1497-84). All mechanical tests on uniaxial tension were performed in a servo-hydraulic biaxial test system Instron 8850 with constant loading rate in the range of 0.5 to 10.0 mm/min throughout the experiment at room temperature. The Instron 8850 is intended for static tests on tension, torsion, compression, flexure and combined tests on tension-torsion with the axial force capacity up to ±100 kN, the torque capacity up to ±1000 Nm and fatigue tests with various wave shapes and frequency up to 30 Hz; the loading rate from 0.1 mm/min up to 240 mm/s. T