Sov'yak I. Computational–Experimental Model for Predicting the Effect of Hydrogen Concentration in Metals on the Structural Integrity of Pipelines with Crack-Like Defects

The dissertation is devoted to mathematical modelling and computational-experimental prediction of the influence of hydrogen concentration in metal on the residual durability of a pipeline with a crack-like defect. The relevance of the study is determined by the fact that long-term operation of pipelines transporting hydrogen-containing media leads to the formation of crack-like defects, whose propagation may cause catastrophic pipeline failure. A computational-experimental model for assessing the residual durability of a pipeline with an internal semi-elliptical crack is proposed. The model combines stress-strain analysis in the crack vicinity, mathematical modelling of hydrogen diffusion and trapping in pipeline steel, experimental determination of cyclic crack resistance characteristics depending on hydrogen concentration, and prediction of fatigue crack growth up to the critical size determining the residual durability of the pipeline. A three-dimensional finite element model of a pipeline fragment with an internal semi-elliptical crack subjected to internal pressure has been developed. In the elastic-plastic formulation, the distributions of hydrostatic stresses and equivalent plastic strain were determined. It was established that the most critical zone is located along the crack front, while the maximum hydrostatic stresses occur near the crack tip due to stress redistribution within the plastically deformed region. Increasing internal pressure leads to higher hydrostatic stresses and plastic strains, whereas crack geometry significantly affects their localization. A mathematical model for estimating hydrogen concentration in pipeline steel has been developed, where the total hydrogen concentration is represented as the sum of diffusible and trapped hydrogen. Hydrogen diffusion is described taking into account hydrostatic stresses, while trapping is based on the Oriani model considering the relationship between trap density and equivalent plastic strain. A non-uniform hydrogen distribution with a local maximum near the crack was obtained. At elevated internal pressure, the contribution of trapped hydrogen increases significantly, intensifying hydrogen embrittlement and accelerating crack propagation. Experimental studies established the influence of hydrogen concentration on the cyclic crack resistance of pipeline steel. Increasing hydrogen concentration reduces the threshold and critical stress intensity factor ranges and accelerates fatigue crack growth. Based on computational and experimental results, a model for predicting the residual durability of a pipeline with an internal semi-elliptical crack was developed. The model integrates finite element analysis with hydrogen-dependent cyclic crack resistance parameters. A parametric analysis of the effects of internal pressure, crack geometry, and hydrogen concentration on the number of cycles required for crack growth to the critical size was performed. It was established that increasing internal pressure from 8 to 20 MPa decreases the residual durability by approximately 6–8 times. Variation of the crack shape parameter a/c may reduce the residual durability by up to three times, while a hydrogen concentration of 5–6 ppm decreases it by approximately 85–95% compared with the initial level. The scientific novelty lies in the development of a computational-experimental approach for assessing the residual durability of pipelines with internal semi-elliptical cracks considering hydrogen concentration, internal pressure, and crack geometry. A mathematical model describing the distributions of diffusible, trapped, and total hydrogen in pipeline steel while accounting for the stress-strain state and Oriani hydrogen trapping was proposed. Quantitative relationships between hydrogen concentration, cyclic crack resistance parameters, and residual pipeline durability were established. The practical significance of the research lies in the development of a computational-experimental model for predicting the residual durability of pipelines with crack-like defects operating in hydrogen-containing environments while considering internal pressure, hydrogen concentration, and crack geometry. The proposed approach has been implemented at Scientific and Production Enterprise with Foreign Investments «UKRTRUBOIZOL» LLC for predicting long-term pipeline operation and improving technologies for manufacturing large-diameter pipes.