Использование деформационного критерия трещиностойкости для оценивания остаточного ресурса труб линейной части нефтепровода

Abstract

Для оценивания трещиностойкости труб, изготовленных из пластичной стали, использовать подходы линейной механики разрушения проблематично ввиду несоблюдения условий плоской деформации. Один из вариантов решения проблемы состоит в экстраполяции формул линейной механики разрушения для коэффициента интенсивности напряжений (КИН) на существенно нелинейную стадию деформирования с помощью функций поправки на пластичность. В работе анализируются результаты испытаний на циклическую трещиностойкость образцов из трубной стали на основе нового инженерного подхода. Получены силовые и деформационные характеристики трещиностойкости исследуемой стали. Дан анализ построенных кинетических диаграмм усталостного разрушения и диаграмм циклического упругопластического разрушения образцов, живучести труб из исследованной стали после длительной эксплуатации.

To estimate the crack growth resistance of pipes made of ductile steel using a linear fracture mechanics approach is of problematic character as the conditions of plane strain are not observed. One of the ways of solution to the problem is the extrapolation of the linear fracture mechanics formulas for the stress intensity factor (SIF) on substantially nonlinear stage of deformation by means of the amendment functions on the plasticity. The results of crack growth resistance tests of samples made of pipe steel on the basis of a new engineering approach are analyzed in the paper. The force and deformation characteristics of the crack growth resistance were received. The analysis of kinetic fatigue failure diagrams and cyclic elastic-plastic fracture diagrams for specimens, crack growth life of tubes from the investigated steel after long-term use is presented. Specimen’s crack is oriented in axial and circumferential directions for study of anisotropy properties. The characteristics of a fatigue crack growth of investigated steel after long operation are discussed. The characteristics of cyclic crack growth resistance for pipe steel after long-term (over 30 years) of operation are determined experimentally: at absolute temperature T = 293 K and a critical threshold values of SIF are: Kth = 4,91 MPa, Kfc = 54,75 MPa, the values parameters of equation (6) are equal mk = 1,556, Ck = 4,0 ⋅ 10 −10. The algorithm and procedure of computational experimental evaluation of crack growth life of objects operated under cyclic elastic-plastic deformation, which is based on the use of strain approach - measuring of specimen’s width reduction, charting the cyclic elastic-plastic fracture diagram, for practical application are proposed. Graphical analysis of data according to crack growth life and service term of the tube of oil pipe line with a semi-elliptical longitudinal crack from damage is presented at various levels of pressure in the tube: 1, 2 and 3 MPa.