PSI - Issue 5

Tatyana Tretyakova et al. / Procedia Structural Integrity 5 (2017) 318–324 Tatyana Tretyakova et al. / Structural Integrity Procedia 00 (2017) 000 – 000

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2.2. Test procedure and equipment

The mechanical tests were provided with the servo-hydraulic biaxial testing system Instron 8850 (100 kN/1000 N∙m, 30 Hz) and the electromechanical testing system Instron 5989 (600 kN) at room temperature. The kinematic loading was performed with the constant rate of 9.0 mm/min. The inhomogeneous deformation and temperature fields were estimated by the 3-D digital image correlation (DIC) measurement system Vic-3D and the infrared (IR) system FLIR SC7700M. The experimental Set-up is illustrated in Fig. 2. In order to increase accuracy of the video system and to minimize the periphery region, the set of digital high-resolution cameras with a resolution of 16 Mp (Prosilica) was used. The strain field registration was provided with the capture frequency of 2 fps. The capture frequency of the IR-camera was about 10 fps. The quasi-static loading, the low speed of the Lüders bands propagation and evolution of the postcritical deformation explain not high rate of the shooting. The DAQ device NI USB-6251 (National Instruments) was used to synchronize all elements of the testing and measurement equipment, so the force measured by the load cell was linked to the corresponding images.

Fig. 2. Experimental Set-up: the biaxial servo-hydraulic testing system Instron 8850 ( 1 ), the 3-D digital image correlation measurement system Vic-3D ( 2 ) and the infrared system Flir SC7700M ( 3 ).

3. Results and discussions

3.1. The Chernov- Lüder’s behavior in specime ns with concentrators

The stress-strain curve for the standard dog-bone flat specimens of the carbon steel have the stage of the yield plateau forming, the material hardening stage and the extended stage of the postcritical deformation. Some results of the experimental study of the Lüders bands propagation and the relay-race deformation mechanism during uniaxial tension of cylindrical samples of carbon steel are shown in the paper of Tretiakova and Vildeman (2013). The experimental results of the uniaxial tension tests of the specimens with concentrators illustrated that the deformation diagram had the similar configuration, so the process of the deformation was accompanied by the plastic strain localization. The spatial-time inhomogeneity of the plastic flow manifests in the form of the yield delay effect and we can observe the yield drop. In order to match the evolution of the strain fields with the loading, the load-time curve for the specimen with the rhomb-shaped notch is shown in Fig. 3. The evolution of the inhomogeneous strain fields from the stereo DIC are illustrated in Fig. 4 in terms of the strain rate fields ( ε i ).