Issue 27

T.V. Tretiakova et alii, Frattura ed Integrità Strutturale, 27 (2014) 83-97; DOI: 10.3221/IGF-ESIS.27.10 94 of the bands was observed repeatedly. Further, the changing between the macroscopic localization of the plastic flow and the recovery of strain field homogeneity was registered. Time max yy  , % av yy  , % max yy   , %/s yy   , %/s 7 t 2.22 1.98 1.09 0.17 8 t 2.25 2.02 1.17 0.17 9 t 2.33 2.07 1.74 0.17 10 t 2.40 2.10 1.13 0.17 11 t 2.41 2.12 1.45 0.17 12 t 2.51 2.15 1.13 0.17 Table 4 : Values of strain and strain rate calculated for the time period 7 12 t t  . (a) (b) Figure 15 : Diagrams of axial strain (a) and the axial strain rate (b) during the time period 12 t , 1 7 t t   . Material Softening Stage It is well established that during tensile test of plastic materials the ‘shoulder’ or ‘necking’ effect at the material softening stage or the co-called postcritical deformation stage, which manifests itself as local thinning of the specimen cross-section. When the strain bands’ propagation had faded away, the increase of plastic strain localization occurred in the central part of the specimen. For example, the evolution of the axial strain rate fields at the stage of the ‘necking effect’ initiation is illustrated in Fig. 16, the load level of 4.75 kN. The time gap ( t  ) between captured pictures was 0.15 second. The moment of the angle change between the specimen axis and the strain band is shown. To analyse the spatial inhomogeneity at the stage of the necking effect evolution, the diagrams of axial strain are calculated for the time period * * 1 6 t t  . The time gap ( t  ) between captured pictures was 1.57 second. As shown in Tab. 5, value of the local axial strain rate rapidly increased with the increase of the localized plastic strain value in the central part of specimen gauge.