PSI - Issue 59

Ihor Dmytrakh et al. / Procedia Structural Integrity 59 (2024) 74–81 Ihor Dmytrakh et al. / Structural Integrity Procedia 00 (2024) 000 – 000

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4. Results and discussion 4.1. Experimental determination of fatigue crack growth resistance parameters

Initially, the test results were presented as separate scatter plots of fatigue crack growth rate da / dN versus stress intensity factor K  for specimens with different hydrogen concentrations C H (Fig. 4). These plots demonstrate the visible significant effect of the hydrogen concentration in the specimens on the fatigue crack growth rate, namely: the increase of hydrogen concentration leads to an increase of da / dN at the same conditions of cyclic loading.

1,00E-03

1,00E-04

1,00E-05 da/dN, mm/cycle

CH=0,074 ppm CH=0,361 ppm CH=1,030 ppm CH=5,462 ppm

1,00E-06

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10

15

20

25

30

 K, MPa(m) 1/2

Fig. 4. Experimental data on fatigue crack growth rate da / dN versus stress intensity factor range Δ K at hydrogen concentration C H in metal.

Table 1. Threshold Δ K th and critical Δ K fc values of stress intensity factor range and constants n and C in Paris equation (2) at different hydrogen concentrations C H in the specimens No C H , ppm Δ K th , MPa(m) 1/2 Δ K fc , MPa(m) 1/2 C , (mm/cycle)/(MPa(m) 1/2 ) n n 1 0.074 10.76 25.45 5×10 – 9 2.87 2 0.361 7.13 18.07 6×10 – 10 3.97 3 1.030 7.13 16.73 1×10 – 9 4.03 4 5.462 7.13 16.65 6×10 – 11 5.56 The received data were processed according to the procedure described in Section 2, and threshold Δ K th and critical Δ K fc values of stress intensity factor range and the constants n and C in Paris equation (2) under different hydrogen concentrations C H in the specimens were determined (Table 1). 4.2. Calculation of residual durability of defected pipelines with different hydrogen concentrations in the metal In this study we considered two typical pipelines of different sizes ( D 1 =610 mm, t 1 =11 mm and D 2 =1020 mm, t 2 =14 mm) subjected to the internal pressure of p = 7 MPa. It was assumed here that these pipelines contain the defects of different shapes and locations (Fig. 5). This confirms the results of the operating pipelines inspection which showed that the most characteristic types of the detected defects were the corrosion furrows, pits and cracks. For calculation of the residual durability N f of the given pipelines, all types of these defects were modeled by semi elliptical cracks according to Dmytrakh et al. (2012) and Kryzhanivs’kyi et al. (2019) with different ratios of their half-axis a and c (Fig. 5). Three types of defects were considered: a / c = 0.25; 0.50 and 0.75. The calculations of N f were made using the formula (3) taking into account the data given in Table 1.