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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7

sample tested in an environment of 3% NaCl solution saturated with hydrogen sulfide showed that at the stage of pre-critical crack growth the probability of fact that at this stage the sample will not be destroyed reaches 75 – 80%. Moreover, the period of subcritical crack growth without sample destruction clearly exceeds the mark of 50% probability P(N) (Fig. 5). At the same time, the critical stage of crack propagation is most often reached when the probability of failure of the sample P(N) becomes less than 0.40...0.45 (and the probability of its failure correspondingly increases to more than 0.55...0.60).

P( N ) =0,4

1E-7

0,5

0,6

0,8 0,7

1E-8

0,9

1E-9 da/dN , m/cycle

T p

T c

T i

0 1 2 3 4 5

N , 10 6 cycles

Fig. 5. Corrosion-fatigue crack growth rate da / dN as a function of the number of fatigue loading cycles for sucker rods 22 mm in diameter made of steel 20N2M with distinct zones within which there is a different failure probability P ( N ). Zones T i , T s and T с correspond to the incubation, subcritical and critical periods of crack growth in the rod.

300

P ( N )

250

1

2

1%

200

5%

1%

10%

150

5%

50%

10%

100 s , MPa

90%

95%

50%

99%

50

90%

95% 99%

0

10 3

10 4

10 5

10 6

10 7

10 8

10 9

N , cycles

Fig. 6. Fatigue life curves σ – N for different probability of non-destruction Р ( N ) of steel (solid lines) and fiberglass (dotted lines) sucker rods with a diameter of 22 mm, tested at circular cantilever bending in formation water. The numbers near the curves indicate the probability of non destruction of the sucker rod in %. Recently, in order to reduce the negative impact of corrosive active components of technological environments on the fatigue life of sucker rods for oil production, they are increasingly made of polymer composite materials reinforced with glass, carbon fibers or in a hybrid version Kret et al. (2020) and Kopey 1 et al. (2020). However, the experience accumulated as a result of the operation of steel rods and laboratory studies of samples cannot be directly transferred to the prediction of the residual life of rods made of more modern materials under different modes of their loading. Separate studies are needed to confirm or refute the patterns of initiation and growth of cracks installed on metal rods. In order to substantiate the workability of modern sucker rods made of polymer composites, their fatigue life was evaluated on sucker rod samples under cyclic loading by circular cantilever bending and the corresponding fatigue curves were obtained. Based on the results of these tests, probabilistic curves of fatigue life of samples with steel and fiberglass sucker rods in the technological environment of formation water were constructed with different probability of their non-destruction P ( N ) (Fig. 6). It is clear that at high loading levels, steel rods have an advantage over composite ones in terms of fatigue life. At P ( N ) = 50%, this tendency is preserved also at stresses commensurate with operational ones (100 MPa). But at lower stresses (close to fatigue life), even a certain advantage of sucker rods made of polymer composites was found over the number of loading cycles until an equivalent