Issue 43

M.P. Tretyakov et alii, Frattura ed Integrità Strutturale, 43 (2018) 146-154; DOI: 10.3221/IGF-ESIS.43.11 149 The value  0 P k is characterized by the absence of a postcritical deformation stage of material and corresponds to the failure when the tensile strength σ В is achieved. And  1 P k is reached, that corresponds to the full implementation of the postcritical stage of deformation and is amounted to decreasing stresses (load) up to zero value by the moment of failure. Diagrams of stretching and unloading of the initial specimens of steel 40Cr in the coordinates "engineering stress - engineering strain" are given in Fig. 2 ( a ). Fig. 2 ( b ) shows pictures of the configuration of the gauge length of the specimens corresponding to the different levels of postcritical deformation and after failure, the red dots correspond to the marks between which the longitudinal displacements and deformations were recorded during the construction of the strain curves. ε , mm/mm σ Р , MPa k Р Level 1 0.136 730 0.04 Level 2 0.171 650 0.15 Level 3 0.194 580 0.24 Table 2 : Characteristic of levels of postcritical deformation achieved at the beginning of unloading. According to the test procedure, the samples with the neck were turned in the test part to a diameter of 5.5 mm, which corresponded to the minimum diameter in the neck of specimen after unloading. The groove of specimens allows us to escape from the geometric nonlinearity of the test part, caused by the strain localization in the neck form. The turning of the specimens was carried out according to the two schemes, the sketches of the samples for which are shown in Fig. 3. a b Figure 3 : Turning schemes of test parts of specimens with neck (dashed lines corresponding to the geometry of initial specimen): a – scheme 1, b – scheme 2 The specimens made according to scheme 1 were used for estimate the evolution of the material mechanical properties in the gauge length of the specimen after necking, and the specimens, prepared according to scheme 2 were used for estimate the mechanical properties of steel directly in the neck zone. Fig. 4 shows the order of specimens processing and testing according to scheme 1 ( a - d ) and 2 ( e - h ). In Fig. 4 ( a, e ) show the initial specimens before testing, after painting for recording by video system the frames for calculating the displacement fields. The pictures b and f in Fig. 4 are correspond to the specimens with strain localization in a neck-form after unloading at the postcritical deformation stage. The picture c in Fig. 4 shows the specimen which is after turning along the entire length of test part according to the sketch a in the Fig. 3 , for further testing according to the scheme 1. Picture g in Fig. 4 presents a sample after turning in the area of the initial neck according to the sketch b in Fig. 3, for further testing according to scheme 2. Pictures d and h in Fig. 4 show the configurations of the destroyed specimens with newly formed strain localization in the neck-form after the tests according to the schemes 1 and 2, respectively. Destruction of all samples made according to scheme 1 occurred in the peripheral part with respect to the necking zone at the initial specimens, as seen in Fig. 4 (b and d). Strain curves obtained for specimens, which made according to scheme 1 and scheme 2, constructed by video system data, are shown in Fig. 5 ( a ) and ( b ), respectively: after unloading from the level 1 (solid line), after unloading from level 2 (dotted line) and after unloading from level 3 (dashed line).