PSI - Issue 42

Halyna Krechkovska et al. / Procedia Structural Integrity 42 (2022) 1406–1413 Halyna Krechkovska / Structural Integrity Procedia 00 (2022) 000 – 000

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according to the calculations by formula (2), the residual resource of the rod at the moment when the defect reaches the critical size of 5 mm will be 8.8 months. After that, the rod should be rejected. The proposed approach is practically important for the effective rejection of sucker rods after their operation in the oil fields. If there are fatigue defects in the sucker rods, their critical sizes depend on the diameter of the rod and vary in range 4...6 mm. Similarly, by applying the proposed expression for determining t rez , it is possible to determine the residual life of rods with fatigue damage of a certain size. It was experimentally established that at the stage of stable (pre-critical) propagation of corrosion-fatigue cracks in samples of rod with a diameter of 22 mm, their growth rate da/dN changed linearly during the tests (Fig. 4). Therefore, it can be described by linear regression equations. Thus, during tests of a rod sample in an environment (3% NaCl + H 2 S) with a stress range of σ = 100 MPa, the rate of subcritical growth of a corrosion-fatigue crack da / dN was described by the following dependence: da / dN = 9.7·10 -9 N / N f + 2.04·10 -10 (m/cycle) (3) While during the research in a 3-% aqueous solution of NaCl without its saturation with hydrogen sulfide for the same range of stresses during the tests, the speed da / dN was described by a linear dependence: da / dN = 5.6·10 -9 N / N f + 8.47·10 -10 (m/cycle) (4) For a rod sample, the surface of which was additionally mechanically treated with a metal brush, tested in an environment (3% NaCl + H 2 S) with a higher range of stresses σ=150 MPa, the rate of subcritical growth of a corrosion-fatigue crack in it was described by the dependence: da / dN = 2.3·10 -9 N / N f + 2.02·10 -10 (m/cycle) (5) At all these dependencies N and N f denote the number of load cycles of the samples from the rods at the current moment and at the moment of their destruction, respectively.

1E-07

( N/N f ) с2

( N/N f ) с1

1E-08

da / dN , m/cycle

1

2

( N/N f ) с3

1E-09

3

0,2

0,4

0,6

0,8

1,0

N / N f

Fig. 4. Influence of the fatigue life exhaustion factor N / N f on the growth rate of a corrosion-fatigue crack da / dN in steel 20N2M of sucker rods with a diameter of 22 mm (without (1) and after (2, 3) treatment of their surfaces by rotating metal brushes), tested at stresses of σ = 100 (1, 2) and 150 (3) MPa in 3% NaCl water solution with additional saturation (1, 3) or without saturation (2) with hydrogen sulfide. It is important to note that surface treatment of sucker rod samples with metal rotating brushes made it possible to slightly increase their relative fatigue life N / N f at the stage of pre-critical crack growth. Moreover, it turned out that the positive effect of such surface treatment is realized even with a higher range of loads (σ = 150 MPa) during th e tests than applied to samples with an untreated surface (σ = 100 MPa). Thus, the critical values ( N / N f ) c of fatigue life for these two variants of sucker rod samples reached 0.75 and 0.87, respectively, from the full resource of the sucker rod N f . Using the proposed approach, the parameters of the fatigue life of sucker rods were also evaluated. They were used as the basis of the methodology for calculating the full and residual resources of sucker rods. The analysis of the dependence of the corrosion- fatigue crack growth rate for the range of stresses σ = 100 MPa in a rod