Issue 51

A. A. Lakhdari et alii, Frattura ed Integrità Strutturale, 51 (2020) 236-253; DOI: 10.3221/IGF-ESIS.51.19 237 graphs of these values according to the thickness of the wall of the cylinder at different times. It is shown that the ANSYS software package adapted to the resolution of such problems can model the behavior of different structures in a hydrogen- containing environment, taking into account the effects caused by both the influence of hydrogen on mechanical properties of the material and by the stress state of the structures, as well as by the influence of the stress state on the interaction kinetics of hydrogen with the structures. K EYWORDS . Hydrogen embrittlement; Hollow cylinder; Physical nonlinearity; Hydrogen effect; Finite element modeling. I NTRODUCTION he influence of hydrogen on the mechanical properties of structural material is a topical problem, because in recent times hydrogen is widely used in various industrial fields. Hydrogen has a destructive effect on materials and structures at elevated temperatures and pressures, as well as at normal temperatures known as conventionally low temperatures [1-6]. At the present time, the term "embrittlement by hydrogen" is understood to include all the detrimental influence of hydrogen on the properties of materials. Hydrogen embrittlement (HE) is a direct consequence of a critical local hydrogen concentration in a metallic material. There are two main mechanisms of hydrogen-assisted rupture [7-9]: - HEDE (Hydrogen-Enhanced DEcohesion); - HELP (Hydrogen-Enhanced Localised Plasticity). The hypothesis of the HEDE mechanism is based on the favoring of the formation of microcracks following the reduction of the cohesion of the metal network, that is to say on the weakening of the inter-atomic bonds, following a high concentration of hydrogen in crack tip. In contrast, the HELP mechanism is based on the ductile nature of hydrogen assisted rupture. The HELP mechanism involves hydrogen-plasticity interactions of an elastic nature. The HELP model represents variations in crack tip plasticity and the HEDE model explains the speed of propagation of microcracks. There is also another Adsorption Induced Dislocation Emission (AIDE) model, which suggests, as with the HEDE mechanism, that hydrogen embrittlement is due to the weakening of inter-atomic bonds. As is known, hydrogen interacts differently with metals, depending on the temperature and pressure exerted on the structure. The introduction of hydrogen into metals and alloys can take place by one of two qualitatively different mechanisms [10]: 1.As a result of low temperature electrochemical processes with the participation of hydrogen ions, which are reduced and absorbed by steel. At low temperatures (i.e. ordinary temperatures), hydrogenation of the metal occurs, resulting in embrittlement by hydrogen and modification of the mechanical properties of the metal. This process is often called low temperature hydrogen embrittlement. 2. From a hydrogenated gaseous medium, at temperatures (above 200°C) and high pressures, as a result of thermal dissociation of hydrogen molecules, forming atomic hydrogen absorbed by steel, which interacts with carbides. High temperature hydrogen corrosion occurs which destroys the material of the structural elements. Many studies have been devoted to the problem of high temperature hydrogen corrosion of metal structures [7-10]. This process is called high temperature hydrogen corrosion. According to the work [11], there are two main lines of research on low-temperature hydrogen embrittlement: a) study of the fundamentals of hydrogen embrittlement processes ; b) development of deformation models and estimation of the longevity of structural elements interacting with hydrogen. The first axis has been studied and analyzed in sufficient detail in many studies and it has been found that the processes of hydrogen embrittlement and degradation of the mechanical properties of pipeline materials were not sufficiently studied [12-16]. T