наведено аналіз результатів багаторічного технічного діагностування апаратів нафтопереробного комплексу з ціллю встановлення типопредставника обладнання для розрахунку на циклічну міцність. методом скінченних елементів встановлено вплив місцевого корозійного дефекту, розташованого в області навколо патрубка, на циклічну міцність кожухотрубного теплообмінника при дії робочих навантажень. бібліогр. 12, табл. 1, рис. 7. К л ю ч о в і с л о в а : технічна діагностика, кожухотрубний теплообмінник, місцева корозія, циклічна міцність

We perform the quantitative fractal analysis of the grain boundaries in the microstructure of Ds steel of five oxygen cylinders with a period of operation from 18 to 52 years. Their fractal dimension is determined by the superposition of square cells of different sizes on the image of the selected grain boundary with computing the number of points of their intersection with this boundary (box method). We establish correlations between the fractal dimension of grain boundaries and the impact toughness, fracture pressure, and yield strength. The corresponding regression equations are deduced. We also analyze the fracture mechanisms connected with the fractal dimension of the grain boundaries.At present, it is very important to prevent frequent explosions of the cylinders for energy carriers with high internal pressure, especially of oxygen cylinders. If we neglect the anthropogenic factor, then wear proves to be the main cause of explosions because, in some cases, the cylinders are exploited for 50-60 yr instead of 40 yr [1]. The accumulated practical experience shows that fracture is caused not only by aging of the metal but also by the technology of its manufacturing. The contemporary methods of monitoring do not enable one to estimate the possibility of subsequent operation of the cylinders in due time [2][3][4][5]. A series of works (see, e.g., [6,7]) is devoted to the analysis of the influence of different structural and textural factors on the mechanical properties of cylinder steel and its coercive force. In particular, a parabolic correlation between the coercive force and the increase in internal pressure in the cylinder was revealed and a procedure for the estimation of the stress-strain state and residual service life of the vessels according to its values was proposed in [6]. As shown in [7], the increase in the content of "brittle" textural component {001} ⟨110⟩ is accompanied by the growth of coercive force due to the increase in the number of crystals magnetized in the direction ⟨110⟩. The influence of grain sizes on the degree of damage to cylinders was also analyzed but the structure of grain boundaries was not studied despite the fact that, in polycrystalline metallic materials, it plays an important role in determining the mechanical characteristics [8] because cracks are mainly formed and propagate just on the grain boundaries. The intergranular fracture can also be caused by the brittleness of structural materials [9] and, thus, can be avoided by strengthening the grain boundaries [10] and changing their morphology.The fractal nature of the dislocation structure in steels after controlled forge-rolling was studied in [11] but there are no data on the influence of the tortuosity of grain boundaries on the mechanical properties of the metal of oxygen cylinders. One can quantitatively determine this parameter according to their fractal dimension, i.e., the measure of complexity of the object. Hence, the configuration of grain boundaries is estimated by the fractal

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