Predicting the residual life of pipelines with surface defects has always been associated with time-consuming and high-cost testing of full-size pipes and pipe strings, since it is not possible to implement a biaxial stress-strain state that occurs in the pipe wall under the influence of internal pressure on standard samples. A method and a special form of the sample segment for cyclic testing of pipe steels according to the criteria of fracture mechanics have been developed. A full-thickness sample is cut from the wall of the pipeline under investigation. The minimum geometric dimensions of its working part, associated with the thickness of the pipe wall, are determined and allow modeling the stress-strain state of the pipeline wall loaded with internal pressure in the center of the working part of the sample under uniaxial tension. Applying an artificial surface crack-like stress concentrator in the center of the working part of the sample with a fixed depth and length of no more than 30% of the width of the sample’s working part allows using the "marks" method to evaluate the parameters of the metal crack resistance of the pipeline under cyclic loading conditions. Plotting of kinetic diagrams of fatigue fracture metal, for the pipeline wall allows assessing numerically the effect of duration of pipeline operation in difficult climatic conditions, the changes in the physico-mechanical characteristics of pipeline metal.
The authors also investigated the cyclic crack resistance of metal pipes made of steel of K52 – K54 strength class in the state of delivery and after 45-55 years of operation. The parameters of cyclic loading were taken from the existing oil trunk pipeline and schematized using the "rain" method. The remaining life of the pipeline wall with a 20% depth of grinding and a surface stress concentrator at the bottom of the grinding with a depth of 1.5 mm, a length of 20 mm, and an opening of 0.2 mm is predicted. Dimensions of the surface concentrator are selected from the condition of stable registration by non-destructive testing methods even in the underwater position. Samples from pipes of K60 strength class of controlled rolling were used to evaluate cyclic durability of the wall restored after electric arc grinding by electric arc build-up.
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