Distribution of Hydrogen Concentration in a Compact Specimen Under the Conditions of Electrolytic Hydrogenation
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DISTRIBUTION OF HYDROGEN CONCENTRATION IN A COMPACT SPECIMEN UNDER THE CONDITIONS OF ELECTROLYTIC HYDROGENATION О. Ya. Chepil’1, 2 and I. P. Shtoiko1
UDC 620.197.5^669.788
We propose a computational model for the evaluation of the distribution of hydrogen concentration in a compact specimen under the conditions of its electrolytic hydrogenation. The compact specimen is modeled by a parallelepiped whose surface opposite to the crack tip is immersed in the electrolyte down to a certain depth. The specimen is hydrogenated through this surface. The applied mathematical model is based on the Fick equation with initial and boundary conditions. A constant hydrogen concentration is assigned on the immersed surface of the parallelepiped; on the other surface, this concentration is set equal to zero. The mathematical model is realized by the finite-element method and the distribution of hydrogen concentration over the parallelepiped is determined in the form of a function of geometry and time. Keywords: hydrogen concentration, electrolytic hydrogenation, compact specimen, Fick equation, finite-element method.
Introduction In elements of different structures operating in contact with hydrogen-containing media and subjected to the action of force loads, we observe the formation of hydrogen-mechanical cracks, which lead to the fractures of these structures and, hence, to large-scale accidents and ecological catastrophes. Therefore, the methods aimed at the evaluation of their residual service life are urgently required. In the literature [1–9], one can find the results of experimental and theoretical investigations of the fracture processes running in metal materials under the action of hydrogen-containing media. For the analysis of the influence of hydrogen on the mechanical and strength properties of these materials, especially on the crack resistance under long-term static or cyclic loads, it is necessary to develop the corresponding procedures constructed on the basis of the force schemes of deformation and fracture of specimens with cracks in chambers with hydrogen under controlled pressure. For their realization, it is customary to use compact specimens whose surfaces are, respectively, loaded, electrolytically hydrogenated, and opposite to the crack and hydrogen chambers (Fig. 1) [6]. The aim of the present work is to determine the distribution of hydrogen concentration in the specimen in the process of its diffusion toward the crack contour. We consider a specimen in the form of a parallelepiped 30 × 50 × 5 mm in size immersed down to a depth of 5 mm in a hydrogen-containing medium (Fig. 2а). In order to apply the finite-element method, the specimen is decomposed into 9000 octahedral elements (Fig. 2b). The problem is solved by using the MSC Marc Mentat 2014.0.0 software package. Under the assumption that a crack is initiated at a point O (Fig. 2а) after a certain time, we determine the behavior of hydrogen concentration in a neighborhood of this point. For this purpose, 1 2
Karpenko Physicomechanical Institute, Ukrai
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