ESTIMATION OF STRUCTURAL SENSITIVITY OF PIPE STEELS ULTIMATE STRAIN TO PIPELINES SERVICE LIFE USING COMPLEX ENERGY CRITERIA OF SYNERGY FRACTURE

The article gives an overview of well-known hypotheses and theories on ultimate state of metals. Processes in metals when stress is applied are described. Multifunctional equation for ultimate strain dependence from density (opening) is offered with all the factors of metal’s ultimate state before fracture. Phases of the fracture of deformed polycrystal structure are specified. An equation is presented that relates the ultimate deformation of a metal to fracture with all the characteristics that determine the limiting state of the metal. Two- or three- compound energy failure criteria equations used for quality estimation of metal structure and performance when main pipeline operating are given for such criteria as energy capacity, crack initiation, crack proliferation, fragility and scale. The fracture criteria calculation for Kh70 steel samples of pipes after operation for 5, 20 and 24  years was made as an example. The comparative analysis of complex fracture criteria values for Kh70 steel pipelines of different lifespans has been carried out. After 5, 20 and 24 years of usage mechanical characteristics of steel Kh70 of main pipelines haven’t changed dramatically, and fracture criteria are inclined to decrease from 21 to 48.5  %, crack proliferation and fragility criteria are the most changeable. At mesolevel the nature of crack initiation is the same for all classes of steel. Under the influence of operating loads, energy intensity, i.e. the ability of metal to withstand any loads with increasing service life is significantly reduced, which is confirmed by the change in the fracture criteria considered in the article. It is shown that the fracture criteria can be used to predict the performance of pipeline steels after a long period of operation.

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