Малоциклова міцність трійника трубопроводу з пошкодженнями

Abstract

Методом скінченних елементів (МСЕ) отримано дані про конструктивну концентрацію напружень та концентрацію напружень залежно від геометричних параметрів об'ємного поверхневого дефекту (ОПД) і місця його розташування на поверхні рівнопрохідного трійника зі сталі 10. Проведено циклічні гідравлічні випробування трійника зі штучно нанесеними ОПД, що імітували корозійно-ерозійні пошкодження. Експериментально встановлено відмінності при статичному та циклічному деформуванні металу в характерних зонах трійника, які зумовлені структурною анізотропією деформації металу. Фактичне число циклів до утворення наскрізної тріщини в зоні дефекту задовільно узгоджується з результатами прогнозу з використанням експериментальних кривих втоми металу трійника, а також і з результатами фрактографічних досліджень.

Stress-strain state and low-fatigue strength of full-scale sample of T-joint with artificial volumetric surface defects under hydraulic inner pressure loading as well as mechanical, cyclic and structural features of material from its different zones have been investigated. Using finite element method (FEM) data on structural stress concentration and stress concentration caused by geometric parameters of the defect and its location on the surface of flush T-joint made from steel 10 were obtained. The tensile and low-cycle tests of material were carried out. The difference in deformation of metal from typical areas of T-joint under static and cyclic loading was established. It is caused by variant morphology of the metal microstructure (structural anisotropy of deformation) in its specific areas that was formed during manufacturing process of the T-joint. Differences of metal microstructure in characteristic areas of T-joint resulted in strengthening and significant loss of the plasticity in one of the zones and corresponded reducing (4..5 times) of the cyclic durability of this area relatively to other investigated areas. Cyclic hydraulic test of T-joint with the artificial volumetric surface defects that simulated corrosion-erosion damage was fulfilled. It was revealed that strain in the defect located along the coupling line of main pipe and branch in the area of structural stress concentration under internal pressure is 2 – 3 times higher than strain in defect with the same geometric parameters at the surface of straight pipe. On the base of investigation of the defect fracture area the mechanism of crack propagation was established. It includes not only formation of facets of brittle and quasibrittle transcrystallite fracture, but regions of the fatigue grooves as well. Number of the groove steps corresponds to the number of cycles for formation of through wall crack in the defect. The failure number of cycles in the area of the defect is satisfactorily consistent with the predicted results using experimental and calculated fatigue curves on the one hand, and results of fractography on the other. It was shown during assessment of the damaged T-joint mechanical properties of metal in its characteristic areas, structural stress concentration and the concentration of stress caused by the presence of defect must be taken into account.