Issue 42

D. V. Orlova et alii, Frattura ed Integrità Strutturale, 42 (2017) 293-302; DOI: 10.3221/IGF-ESIS.42.31 294 is valid only for certain chaotic distributions of dislocations, when their density is considered to be independent of coordinates [10]. That is, beginning with the Griffith’s [9] hypothesis about the existence of places for the initiation of cracks, deformation criteria consider local characteristics for the development of cracks and the structural parameters of materials. Also, the microscopic approach was mainly used to consider the physics of material fracture. Therefore, the complete description of a plastic flow only on the basis of micromechanisms is impossible. Thus, inefficient dislocation models in the physics of strength and plasticity were replaced with the complex concept of a multilevel approach. A multiscale approach to the problems of plastic deformation and fracture was developed due to the works of domestic and foreign researchers [11-19] and currently is used by the scientific community. Thus, at present a deformable material is considered to be a nonlinear medium, an appropriate description of which is impossible without the approach based on the concepts for the mechanics of nonlinear media, synergetics and nonlinear acoustics. So, using the synergetic approach describing the self-organization of highly nonequilibrium systems, the authors of this article and coworkers found a new type of wave processes in a solid. Any plastically deformed object is known to be under nonequilibrium conditions [20]. In order to describe strain, it is proposed to consider a deformed macroobject to be an active medium that includes two factors: activator and inhibitor. Moreover, localized shear processes are considered to be an activator, and the redistribution of local stresses connected with these processes is considered to be an inhibitor. This was the basis for the autowave model for the evolution of localized macrodeformation [21-24]. In contrast to ordinary waves described by hyperbolic equations, autowaves are the solutions of parabolic equations, an important feature of which is the presence of a time derivative, which determines their applicability for the description of reversible processes [25-26]. Autowaves are formed only in the media with energy dissipation and when there are external nonperiodic effects. From this point of view, the strain of any object represents the evolution of autowaves during different type deformation. The change in the type of autowaves is determined by the law of strain hardening, that is, it depends on the stages of the strain flow curve (rule of correspondence). Thus, the movement of a solitary deformation front is observed within the yield plateau (the work hardening coefficient 0   ) along the sample, the so-called autowave excitation or switching. At the linear stage ( const   ) there is the coordinated movement of localization zones (phase autowaves). The stationary structure of zones (stable dissipative systems) corresponds to the stages of parabolic hardening   1 2 1 2 ~ , ~      . A stationary high-amplitude zone (domain) of localized plasticity is formed in the sample at the place of future fracture at the final stage of the deformation process (pre-fracture stage), and the movement of other localized plasticity domains is nonuniform but self-coordinated. This article is devoted to deep studying the macrolocalization of plastic deformation at this stage. In addition, the authors of this article believe that a complex approach based on a combined analysis of the plastic flow localization during the deformation process and supplemented by the simultaneous measurement of small changes in the velocity of ultrasound propagation should be applied to the problem concerning the reliable estimation of the mechanical condition and the prediction of the resource for structures in the long operation. This will allow us to determine the exact location of the places with damages and find a connection with a localized plastic flow. Recently, an acoustic analysis of materials under loading has been developed in the field of plastic deformation in theoretical and experimental studies by Kobayashi [27, 28], Maurel [29], Zuev [30]. At present, individual groups of researchers only start developing such an approach, mainly by using idealized simple model media. For real materials used in technological production and materials with a complex structure, such an integrated approach has not been developed yet. Thus, the aim of the work is to study the stage of pre-fracture for iron based alloys and analyze the possible use of an autowave model to formulate criteria for the structural strength of materials and elastic-plastic transition. The last fact will allow an important technological problem to be solved and the behavior of structural materials to be controlled before the initiation of irreversible plastic deformation. Since the inhomogeneity or localization of deformation is manifested not only in the accumulation of defects during the operation of constructions, but also in many technological processes connected with pressure treatment, drawing, etc., the estimation of material plasticity is an urgent problem for these processes. M ATERIALS AND EXPERIMENTAL PROCEDURES teel of different composition was chosen as test materials. Heat-resistant stainless steel (AISI 420), cryogenic structural steel (321N), carbon steel (G10080), and electrical steel (alloy of iron and silicon) were used. These iron based alloys are widely used not only in industry, but in experimental studying the physics of plasticity and strength. S