PSI - Issue 6

Ekaterina L. Alekseeva et al. / Procedia Structural Integrity 6 (2017) 128–133 E.A. Alekseeva et al. / Structural Integrity Procedia 00 (2017) 000–000

3

130

Fig. 1. (a) The distribution of the acoustic anisotropy a along the axis of the corset sample; (b) Distribution the concentration of dissolved hydrogen along the axis of the corset specimen.

Fig. 2. A photograph of a cylindrical specimen of alloy of EP718 alloy after failure with clearly visible bands of plastic strain localization.

This important result requires discussion of the dimension problem. Real designs usually have a three-dimensional structure of all critically stressed elements. They are equipped with sti ff ening ribs and plates. Under these conditions, experiments with plane samples do not always adequately describe the nature of deformation of real structural details. Moreover, when the mechanism of the observed phenomena is debatable. In particular, the nature of the formation of plastic strain localization bands in three-dimensional samples has not been described in the literature. We chose an alloy of EP718 alloy (45 Ni Cr Mo W Ti Al B) with grain sizes from 20 to 600 µ m as a model material. Experiments on uniform tension of cylindrical specimens showed that well-marked bands of localization of plastic deformation are formed on three-dimensional specimens. The sample after failure with clearly visible bands of plastic strain localization is shown in Fig.2. Thus, the experiment shows that the localization of plastic strains is a universal process that precedes failure. The method of acoustic anisotropy allows e ff ectively detect the failure that gives new means of technical diagnostics. There remains a discussion concerning mechanism for the loss of stability of deformation under uniform tension of the samples.

3. Model

In order to take into account the influence of the main factors on the inhomogeneity of plastic deformation, let us consider the simplest model for the inhomogeneity of the material. It is known that the deformation of a polycrystalline metal during pressure treatment occurs as a result of plastic deformation of each grain. And since the grain sizes di ff er and they are not oriented in the same way, the plastic deformation can not proceed simultaneously and equally throughout the entire volume of the polycrystalline material.