In order to determine the effect of the plastic zone on the fractal properties of a fracture surface, specimens of medium-carbon steels are studied after tough and brittle fracture. A fractal analysis is conducted for each specimen by the method of vertical sections. The sizes of the determining parameters of the fractures and plastic zones under the fracture surfaces are determined by measuring the Knoop microhardness.Despite the growing number of publications devoted to fractal analysis of fractures that appear predominantly in foreign literature, stable dependences relating the fractal dimension (fracta! number) that characterizes the geometry of a fracture with the mechanical properties of the material have not been derived yet. Such dependences can be found by studying the interrelation between the variation of the fractal number and of the zone of plastic deformation under the fracture. Indeed, the zone of plastic deformation is a conductor of the effect of the structure of the material on its operating capacity and mechanical properties, because the structural sensitivity of the material is determined by the ratio of the size of this zone to the size of the structural elements.A complete description of the relation between the fractal dimension and the structure of the material and the type and parameters of the fracture is given in [1]. However, this work does not evaluate the size of the plastic zones. A supposition that the equality of the measuring step 8 of the fracture profile and the size of the plastic zone should lead to the formation of a plateau on the Richardson curve relating the length of the profile L with the measuring step ~5 has been made in [2]. It is possible that ~e establishment of the relation between the size of the plastic zone and the behavior of the Richardson curve will allow us to understand the structural features of the development of fracture in a material and the causes of changes in the fractal dimension.The aim of the present work consisted in analyzing the effect of the size and nature of the zone of plastic deformation on the form of the Richardson curve. For this purpose, we compared the results of a fractal analysis of the profiles of fracture surfaces for Charpy specimens (I and 2 ) tested for impact bending, and the profile of an operational crack formed in a roller bit (specimen 3 ) with the structure of the metal, the results of qualitative and quantitative ffactographic analyses, and the values of Knoop microhardness of the material measured under the fracture surface for evaluating the length of the zone of plastic deformation.Specimens 1 and 2 were fabricated from medium-carbon steel 30KhMA, and specimen 3 was made of steel 14N4. The chemical composition and the mechanical properties of the steels are presented in Table 1.The results of the metallographic and fractographic analyses of specimens 1 and 2 from steel 30KhMA are presented in Figs. la-ld. Steel 30KhMA was subjected to a heat treatment that consisted of normalizing from 870~ quenching from 770~ in wat...
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