Mykhailo Hrynenko

The results of an experimental study on the effect of preliminary elastoplastic deformation and hydrogen charging of steel 45 on the fragmentation intensity of cylindrical shell struc¬tures under internal explosive loading are presented. The preliminary elastoplastic defor¬mation was carried out by pressing a steel ball into a hole along the axis of a cylindrical specimen, creating local plastic zones and damage. Part of the specimens, after deformation, were subjected to hydrogen charging in a special chamber to reduce the material plasticity. Prepared hydrogen-charged and check (uncharged) specimens were filled with explosive charges and tested at a polygon. The results showed that the specific fragmentation para¬meter for hydrogen-charged specimens was more than 2.4 times higher than that for un¬charged specimens. The proposed approach provides a simple, technology-based method for controlling shell fragmentation without complex geometric stress concentrators. A number of fragments and their weight was analyzed.

Methods of deformation of cylindrical shells made of steel 45 are proposed, and the influence of residual deformations on the concentration of gaseous hydrogen absorbed by the material is evaluated. Deformation of cylindrical shells is implemented by pressing a sphere into the hollow cylinder under compression and tension. The features of each method are identified by monitoring the deformation dynamics of the samples using an optical-digital system. To compare the degree of deformation, the residual equivalent deformations of the samples are calculated. It is found that at a preliminary 21%-deformation of cylindrical shells, the concentration of hydrogen absorbed by the material increases in three times compared to undeformed material. During compression deformation of the samples, a slight increase in the concentration of absorbed hydrogen is observed as com¬pared to tension deformation for the same equivalent deformation.

Tensile tests of the alloyed 60S2А spring steel were carried out under the action of gaseous hydrogen of increased parameters: temperature 400°С and pressure 6 MPa. True fracture diagrams of 60S2А steel were constructed for different hydrogenation time. To determine the value of local deformation, the method of optical-digital correlation of images on the surface of the sample and special software were used. It was established that with increasing hydrogenation time, the destructive stresses and deformations decrease according to a linear law.

This paper presents the results of an experimental study on the effect of preliminary elastic–plastic deformation and hydrogen charging of steel on the fragmentation intensity of cylindrical shell structures under internal explosive loading. The preliminary deformation was carried out by pressing a steel ball along the axis of a cylindrical specimen, creating local plastic zones and damage. Part of the specimens, after deformation, was subjected to hydrogen charging in a special chamber to reduce the material’s plasticity. Prepared hydrogen-charged and control (uncharged) specimens, in the form of scaled projectile models, were filled with explosive charges and tested at a proving ground. The results showed that the specific fragmentation parameter for the hydrogen-charged specimens was more than 2.4 times higher than that of the uncharged ones. The proposed approach provides a technologically simple method for controlling the fragmentation of shell structures without the use of complex geometric stress concentrators.

A method for assessing the stress-strain state in cylindrical shells was developed, taking into account the effect of external force loads and the influence of absorbed hydrogen. This influence promotes an increase in internal damage to the metal and a significant rise in the concentration of absorbed hydrogen, leading to the overall embrittlement of the metal. As a result of reduction of the specific fracture energy the material damage resistance decreases. The finite element method and the digital image correlation method was employed to analyze the distribution of axial, tangential, and radial stresses in the walls of cylindrical shells, as well as their effect on the overall deformability of the structures. Special attention was paid to the influence of prior material deformation and hydrogen degradation on the material resistance to fracture. The results showed that hydrogenation could significantly reduce critical strain, accelerating the fracture process, especially at high hydrogen concentration. Pri-strain plastic deformation contributes to an increase in internal damage in the metal and a substantial rise in the concentration of absorbed hydrogen, leading to the overall embrittlement of the metal and the specific fracture energy decrease.

The problem of hydrogen diffusion in a metal with pores is formulated. Pores are sources of absorption (traps) of hydrogen. Traps were considered to be point-like, distributed uni­formly over the entire volume of the metal. Solutions to the problem of diffusion in a defective body have been obtained. In particular, expressions for determining the concen­tration of both diffusion-moving and trapped hydrogen are obtained for the plate. We also obtained a ratio for determining the amount of hydrogen sorbed by a defective plate at an arbitrary moment of time during its hydrogenation. The amount of hydrogen sorbed by a defect-free plate and a plate with uniformly distributed traps was compared. A 2-3 time excess of hydrogen content in the defective plate compared to the defect-free plate was established.

It was established that the metal of long-term operated gas pipelines made of 10G2BT steel contains relatively little hydrogen, which does not create prerequisites for the pipes hydrogen embrittlement. However, for the considered cases, it was established that the microstructure of steel is characterized by increased defectiveness, which increases with the extension of the pipeline operation time. Such a defective microstructure has an increased ability of absorbing hydrogen, and therefore there is a risk of hydrogen embrittlement of the material. It was concluded that the use of long-term operated pipelines for transportation mixtures of natural gas and hydrogen without additional verification is problematic.