The Spatial-Time Inhomogeneity of the Plastic Flow in Metals by Analysis of the Strain and Temperarure Fields
The work focused on the complex experimental study of the behavior patterns of the Al-Mg alloy (Al — 2.2 %, Mg — 0.6 %, Mn — 0.4 %, Fe — 0.4 %, Si — 0.4 %) and the carbon steel (Fe — 99.0 %, C — 0.18%, Si — 0.20 %, Mn — 0.35 %, Cr — 0.04 %, Ni — 0.03 %, Cu — 0.04 %), in particular, the temporal instabilities and spatial localization due to the Lüder’s behavior, the Portevin–Le Chatelier effect and the shoulder or necking effect. The aim of the study is to assess the influence of the loading conditions, the stress concentrators and the additional cyclic impact on the effects of the serrated flow, on the initiation of the strain bands, and to interrelate local strain jumps with temperature bursts. The study based on the estimation of the inhomogeneous strain and temperature fields by use of the 3D digital image correlation measurement system Vic-3D and the infrared analysis system FLIR SC7600M. The mechanical tests were provided on the Instron 8850 servo-hydraulic biaxial testing system (100 kN/1000 N∙m, 30 Hz) and the Instron 5989 electromechanical testing system (600 kN) at room temperature.
The research program included the following parts:
- the uniaxial tension of the flat dog-bone specimens with the constant rate of the kinematic loading from 0.33∙ 10−4 s−1 to 0.33 × 10−2 s−1 (steel);
- the uniaxial tension of the flat specimens with one and two holes with constant rate of the kinematic loading 1.67∙ 10−3 s−1 (steel);
- the uniaxial tension of the flat specimens with additional deformable parts and the flat specimens with complicated geometry with constant rate of the kinematic loading 1.67∙ 10−3 s−1 (Al-Mg alloy);
- the uniaxial tension with additional cyclic loading of the solid cylindrical specimens (steel).
For example, the σ–ε curve and the set of photos of the flat specimen during the Lüder’s band nucleation (Fig. 1) and the evolution of the local strain rate and temperature fields on the surface of the flat specimen with two holes (Fig. 2) are illustrated below.
Fig. 1. Evolution of local strain rate fields at the stage of the yield plateau formation on the surface of the flat dog-bone specimen of the Al-Mg alloy
Fig. 2. Initiation and propagation of the strain bands on the surface of the flat specimen (the carbon steel) with two holes