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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loading diagram. As shown in Fig. 3, at the initial deformation stage the plastic flow localizes near the concentrator (stage I ), with further loading at the moment of the switching from the upper to the lower yield point on the surface of the specimen the X-shaped bands initiate (stage II ). The X-shaped bands of the localized plastic flow propagate as shown in Fig. 3. The stage II is similar to the stage of the yield plateau forming, when the Lüders bands propagate along the gauge length of the flat dog-bone specimen. At the point 4 the movement of the bands stops, the active plastic deformation starts upper and below the concentrator, and again near the notch (stage III ). It is important to note that the load decreases at the point 4 , such drop is observed before the material hardening stage at the standard stress strain curve. Starting from the point 5 , the plastic flow localized near the concentrator (stage IV ). The active plastic deformation accompanies by the damage accumulation and at point 6 we can observe the crack initiation (stage V ). For the specimen with concentrator it is the beginning of the postcritical deformation stage. The similar analysis was provided for the specimens with a single hole and a central crack-like notch. For example, the Fig. 5 shows the evolution of the strain rate fields for the sample with crack-line notch. It has to be pointed out that the process of the plastic deformation has the same stages.

Fig. 5. Evolution of the inhomogeneous strain rate fields on the surface of the specimen with the rhomb-shaped notch during uniaxial tension.

The experimental results from the IR-camera are processed in the form of the time dependent curves of the maximum temperature on the specimen surface. These graphs show the correspondence to the stages on the load-time curves. The initiation of the X-shaped bands causes the increase of the temperature (stage II , Fig. 3). During the evolution of the plastic deformation of the material upper and below the concentrator (in the peripheral area) the maximum temperature decrease. The strain rate in the bands of localized plastic deformation is significantly higher than in the peripheral area. The next increase of the temperature on the specimen surface exhibits at the stage V (Fig. 3) and can be explained by the crack propagation.

3.2. The evolution of the postcritical deformation zones in specimens with concentrators

Based on the analysis of the experimental data (stress-strain curves), collected from the set of the uniaxial tension tests of the standard flat dog-bone tensile specimens, it is obtained the value of the macroscopic strain of ε 20.5 i  %, which corresponds to the moment of the transition to the postcritical deformation stage. It is illustrated in Fig. 6 (a) with a vertical dashed line. This data was used in order to indicate the real configuration of the postcritical deformation zone in the specimens with concentrators.