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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3. Material and experimental procedure For present study the typical ferrite-pearlite pipeline steel ( σ Y = 260 MPa and σ U = 440 MPa) with nominal chemical composition (in weight %): C = 0.17 – 0.24; Si = 0.17 – 0.37; Mn = 0.35 – 0.65; S < 0.04; Fe balance, was chosen. The rectangular cross-section beam specimens (Fig. 2) were manufactured with real pipes. The initial edge crack of length a 0 ≈ 3.0 mm was nucleated in all specimens by cyclic pure bending with frequency f = 1 Hz under stress ratio R = 0 in the air at ambient temperature. The stress intensity factor K I in the crack tip for the specimen (Fig. 2) in the case of its loading by pure bending was calculated from the known formula according to Dmytrakh (2011).

A

3mm

20 mm

Initial crack

10 mm

150 mm

A

1 mm 60 0 R=0.1mm

Fig. 2. Beam specimen with initial fatigue crack.

Before testing the specimens were preliminary hydrogen charged to the different values of hydrogen concentration C H in them (Dmytrakh et al. (2015)). For this, they were saturated by gaseous hydrogen in a special chamber at the pressure of 5.5 MPa and a temperature of 400°C (Fig. 3a). a b

Fig. 3. Special chamber for hydrogen charging of specimens (a) and complex of LECO DH603 hydrogen analyzer (b).

The hydrogen-charged specimens with initial edge crack of length a 0 ≈ 3.0 mm were subjected to cyclic pure bending with frequency f = 1 Hz under stress ratio R = 0 in air. The crack lengths were measured on a plane surface of specimens by the optical microscope with accuracy of ± 0.005 mm. Resulting data at different hydrogen concentrations in the bulk of specimens were received as the sequence of the following parameters: number of loading cycles N i , crack length a i , fatigue crack growth rate ( da / dN ) i , and stress intensity factor range Δ K i . For the determination of the hydrogen content in the specimens, the LECO DH603 hydrogen analyzer (Fig. 3b) was used, which provides accurate measurement of C H values by LECO DH603 (2019). This is a commercial device (LECO Corporation, USA). Here it should be noted that for each series of tests we used two identical specimens. The first specimen was used for hydrogen content measurement in the specimen before its fatigue testing and the second one was used for fatigue crack growth rate determination.