Syrotyuk A. Methods for assessment of resistance to fracture and strength of pipelines under action of corrosive and hydrogenating environments

The aim of the thesis - development methods and establishment of basic parameters for fracture process evaluating and strength forecasting of pipe steels and constructions based on the concepts of fracture mechanics of materials with taking into account the influence of corrosion and hydrogen factors. Object of study: the processes of fracture of pipe steels that realize by the way of initiation and propagation of crack-like defects under conditions of joint action of static and cyclic loads and corrosive and hydrogenating environments. The methodological basis of the work: the experimental and calculation approaches of fracture mechanics and strength of materials. Beside that the methods of physical chemistry, physics of solids and materials science were used. The stages of initial corrosion damaging and surface corrosion fatigue nucleation in pipeline steels are considered. The effects of chemical composition of environment, frequency and stress ratio and operation time on fatigue corrosion crack growth resistance of pipeline steels are shown. The characteristic values of hydrogen concentration in pipeline steels on dependence of applied stresses and physico-chemical conditions of hydrogenation are determined. The relationship between fatigue crack growth rate and local hydrogen concentration at crack tip is established for pipeline steels based on experimental data on crack propagation and results of measurement of the local hydrogen concentration at the crack tip under different hydrogenating conditions. The formula for determination of crack growth rate as a function of local hydrogen concentration in the crack tip, hydrogen concentration in metal bulk and stress intensity factor range is derived. On this base the new methods for strength assessment of pipeline steels in hydrogen-contained environments are proposed. For expert assessment of serviceability and fracture risk of pipelines systems the special diagrams are proposed, which contain three characteristic zones: safe exploitation, exploitation with predicted growth of existed defects and zone of brittle fracture risk. Area of application - in engineering and technical centres and manufacturing enterprises of heat energy, chemical industry and pipeline transport.