PSI - Issue 13

Tretyakov M.P. et al. / Procedia Structural Integrity 13 (2018) 1720–1724 Tretyakov M.P./ Structural Integrity Procedia 00 (2018) 000 – 000

1723

4

a b Fig. 2. The order of testing and turning specimens in according to scheme 1 (a) and scheme 2 (b).

After unloading, the samples were turned to escape from the geometric nonlinearity of the test part, caused by the strain localization in the neck form. The order of testing and turning specimens was carried out according to the two schemes, which are shown in Fig. 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 necking region. The hardness of steel 40Cr in the necking region after failure was measured too. In accordance with proposed test procedure, the experimental data of mechanical properties of steel 40Cr during necking effect is investigated. It is shown, that the values of the elasticity modulus at the initial linear section of the strain curves are not depend of the tentative reached level of postcritical deformation and the scheme of specimens turning. The average value of the Young's modulus determined on the initial specimens was 204 GPa with deviation of experimental data not more than 5 %, and after turning was 174 GPa with deviation not exceed 8 %. The results reflecting the dependence of the ultimate stress upon stretching of samples turned along the entire gauge length (dashed line) and turned in the necking zone (dotted line) versus the degree of achieved postcritical deformation are shown in Fig. 3. As shown on Fig. 3, the strength of steel 40Cr in peripheral parts of gauge length of samples with neck does not depend on level of deformation. However, strength of studied steel directly in neck significantly depends on achieved postcritical deformation level. 4. Test results

Fig. 3. The ultimate stress obtained in tension tests of specimens turned along the entire length of test part (dashed line) and specimens turned in the necking area (dotted line) depending to level of the achieved postcritical deformation.