We present fatigue test results obtained for the 018GS2 steel specimens cut from the "Druzhba" oil pipeline of 630 mm diameter: as-supplied and after long-term service. We founded that, due to the long-term service, fatigue strength of the bulk material decreased by 14.9%, and that of welding joints - by 9.5%. The analysis shows that service operation of linear portions of the oil pipeline is accompanied by continuous and significant deterioration of the material characteristics of the inner pipe surface which controls the pipeline service reliability, one observed in the bulk material results in formation of additional structural elements and decreases the effect of the local stress concentrators. The above behavior is observed for the whole investigated frequency range of cycling loading. Change in the cycling rate results in the insignificant size reduction of microvolumes with maximal structural changes at the stage of fatigue-damage accumulation and of cell-structure elements at the stage of macrocrack propagation.
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