Abstract. In this paper we study the problem of the contact characteristics changing on a decrease of the load applied to the preloaded joint of roughness surfaces. The penetration of a rigid sphere (indenter) into the elastic hardenable half-space is considered originally. The elastic crater restoring by unloading is also considered. In elastic-plastic material's describing, Hollomon's pawer law is used. To describe a contact of a rigid rough surface with an elastic plastic half-space, the discrete model of a rough surface is used. Microasperities are represented as a set of identical spherical segments, the height distribution of which corresponds to the bearing profile curve of the surface. To describe the bearing profile curve, incomplete beta function ratio is used. The relations of relative contact areas η and e η and gap densities at the joint e Λ and ce Λ on dimensionless loading q F and qe F at loading and unloading for different values of y ε and n are given. The obtained results are of practical importance for the sealing ability prediction of fixed sealing joints at the design stage, in particular for tightness supply of flange couplings and high pressure vessels seals.

The paper indicates that the application of roughness models and the theories of contacting rough surfaces developed by Greenwood-Williamson and N.B. Demkin for solving the problems of hermetology leads to significant errors. This is explained by much greater contact pressures than for the tribology problems, by describing only the initial part of the reference surface curve, the lack of allowance for the plastic extrusion of the material. A brief review of methods for describing the introduction of a sphere into an elastoplastic reinforced half-space is given. The properties of the elastoplastic reinforced material are described by the power law of Hollomon. To describe the indentation and flattening of single spherical asperity, the results of finite element modeling are used. The cases of contacting a rigid rough surface with an elastoplastic half-space and a rigid smooth surface with a rough surface are considered. To determine the relative contact area, the discrete roughness model is used in the form of a set of spherical segments distributed along the height in accordance with the curve of the reference surface.

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The article presents data on the mercury content in the muscles of perch Perca fluviatilis (L.) in four large reservoirs of the Vologda region (Kubenskoe and Vozhe lakes, river and lake (Lake Beloe) parts of the Sheksna reservoir) collected during the period 2007–2018. The metal concentration varied widely: from 0,01 mg/kg for perch from Lake Vozhe to 1,51 mg/kg for perch from Lake Kubenskoe. The highest percentage of fish with mercury content exceeding the established SanPiN 2.3.2.1078-01 norms (29,4%) was recorded in perch from Lake Kubenskoe, while the smallest – in fish from Lake Vozhe (5,4%). In fish from the Sheksna reservoir the content of the toxicant in muscles did not go beyond the accepted SanPiN 2.3.2.1078-01 norms. For all perch samples, a statistically significant positive correlation between mercury content and linear-weight characteristics, as well as fish age, was established.

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To study the flattening of the sphere, it is proposed to use the kinetic indentation diagram by the plane. Given the known values of the reduced elastic modulus, applied force, maximum and residual deformation, it is possible to determine the contact area. It is indicated that in this regard, the exponent of the unloading curve of a pre-loaded sphere with a flat rigid surface plays an important role. The analysis of methods for determining the unloading curves of unloading for the finite element models, taking into account strain hardening, is carried out. It is shown that dependences of the unloading curves during flattening on the relative indentation in the form and the range of values differ from the similar ones during indentations of the sphere. The dependence between the exponents of the unloading curves for the force and for the area is determined. The range of correct use of the results of the finite element analysis of a hemisphere for rough surfaces is indicated. The exponent of the unloading curve after flattening the spherical segment from the half-space property is determined.

The article presents information about the mercury content in the muscle tissue of the European grayling Thymallus thymallus (L.) in 11 rivers of the Vologda region (Tagazhma, Vozhega, Votcha, Kostyuga, Tiksna, Sheben’ga, Pyrnug, Zemtsovka, Yurmanga, Bol’shaya Rechka and Yontala) for the period of 2011–2015. The average content of toxicant ranged from 0,04 mg/kg in individuals of the river Sheben’ga up to 0,21 mg/kg in the rivers Yurmanga and Bol’shaya Rechka. Four fish noted an excess of the permissible level of sanitary and hygienic standards (SanPiN 2.3.2.1078-01. 2.3.2.) for non-predatory fish species (0,3 mg/kg). Young grayling contains an average of 0,06–0,10 mg/kg mercury in muscle tissue, individuals five to six years old contain 0,26 and 0,27 mg/kg, respectively. A comparison of the level of mercury concentration in the muscle tissue of European grayling with that of other fish species showed that the Hg content does not go beyond the revealed background values for water bodies of the Vologda region.