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

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cations inside the metal during plastic deformation. A downside of the dislocation model is that the initial dislocation density significantly e ff ects their further formation and motion. Therefore, the dislocation model does not allow us to make a quantitative prediction without preliminary fitting the parameters from the results of testing the material with the same initial dislocation density. Therefore, generalized models of plastic strain localization, which interpret it as a process of loss of stability of motion, are applied. For this, a deformation with a small constant rate is considered, and elements that can cause instability of solutions are introduced into the equations of a continuum see Penning (1972); Lebyodkin (1996); McCormik (1972); Kalk (1997); Leoni (2014). As a rule, the decreasing dependence of the stress rate on the strain rate is used. Despite the good agreement between the results of modeling and experiment, this approach is detached from real physical mechanisms. In the works known to us, the parameters of the descending branch of the stress-strain curve are adjusted to the parameters of the localization bands observed experimentally. At the same time, the formation of plastic strain localization bands is an important diagnostic feature that makes it possible to determine the proximity of the material to failure, since before the destruction these bands tend to merge into one large one. Detection and determination of the parameters of these bands make possible to carry out technical diagnostics and to predict the begin of failure. A promising method of nondestructive testing is the method of acoustoelasticity. A key characteristic of the method is acoustic anisotropy. A number of fundamental works have been devoted to the study of acoustic anisotropy in the field of elastic strains cf. Hughes (1953) and small uniform plastic strains cf. Hirao (1985); Kobayashi (1987); Kamyshev (2017). Despite this, to date, there are no methods for evaluation of the stress-strain state of structures by the acoustoelasticity method at large plastic strains. The results of experimental studies (see Belyaev&Lobachev (2016); Belyaev&Polyanskiy (2016)) show that the strong inhomogeneity in the distribution of the acoustic anisotropy is observed at large plastic strains on plane samples. To analyze the causes of the inhomogeneity in the distribution of acoustic anisotropy we used various ap proaches. All experiments were carried out with plane samples of 15-17 mm thickness and of 200-500 mm length of working zone. Standard industrial equipment and standard techniques were used cf. Belyaev&Lobachev (2016); Belyaev&Polyanskiy (2016) to measure acoustic anisotropy. In the case of elastic strain dominance, the acoustic anisotropy is proportional to the principal stresses. In the case of plastic strain dominance, the acoustic anisotropy can be well measured, but it has been established that it cannot be used to evaluate the principal stresses. For samples with a stress concentrator in the form of a hole, or in the form of a fatigue macrocrack formed under cyclic loading, the plastic strain fields were calculated by the finite element method. The finite element program ANSYS is used in our computations. It provides the exact evaluation of shape and size of large plastic strain. For the plane corset specimens, it was not possible to determine the zones of plastic strain localization and the failure location of by the conventional calculations. For research we use the original method. Technically pure aluminum due to its production technology is saturated by solid solution of hydrogen. Almost all hydrogen in aluminum is located in the pores, microcracks and other struc tural defects placed along grain boundaries cf. Bekman (1981). This allows us to relate the accumulation of defects with an increase in the hydrogen concentration and thus determine the zones of plastic strain localization in aluminum. Measurements of the distribution of hydrogen concentrations were carried out with the help of an industrial ana lyzer of hydrogen AV-1 according to the procedure detailed in Polyanskiy (2007). The results of acoustic anisotropy measurements and hydrogen concentration distribution along the axis of the sample are shown in Fig. 1 (a, b). Similar results were obtained for steel corset specimens, although there is no such unambiguous relationship be tween the concentration of hydrogen and the concentration of defects in steels. Measurements of the distribution of the thickness of deformed samples during tension also correlate well with the curves in Fig. 1. This allows us to conclude that it is possible to detect zones of plastic strain localization by means of measurements of acoustic anisotropy. 2. Experiment