PSI - Issue 26

Myroslava Hredil et al. / Procedia Structural Integrity 26 (2020) 409–416 Hredil et al. / Structural Integrity Procedia 00 (2020) 000 – 000

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state (Fig. 2a) had less developed relief than in the operated steel (Fig. 2b). Cleavage ridges separating adjacent practically parallel surfaces of the local fatigue crack growth were mostly elongated in the main direction of macrocrack growth. These ridges are more appreciable on the fracture surface of the operated steel. The relief along their tops is damaged and covered with oxides (the brightest areas on the Fig. 2). It is suggested that these damages result from the contact of crack edges in each loading cycle. The reason for this, as shown by Ritchie et al. (1982), Student 1 et al., Student 2 et al. (1999), Lesiuk et al. (2020), is the mismatch of microprotrusions and depressions on the conjugate of the fracture surfaces (due to plastic deformation of metal in the vicinity of the crack tip) creating roughness-induced crack closure. It is suggested that the fatigue fracture surface relief of the operated steel is more noticeable due to damages accumulated in it during its long-term service. These microdamages promote the occurrence of plural centers of crack initiation, and propagation planes of these cracks do not always coincide. Thus, the role of crack edges contact is more essential for the operated steel and, consequently, the crack closure effect is more pronounced as compared with the as-received steel.

a b Fig. 2. Fractographic features of near-threshold fatigue crack growth in air for 17H1S steel in the as received state (a) and after long-term operation (b).

2.2. Effect of corrosive environment

Steel 17H1S. Fatigue testing in NS4 solution was carried out with a frequency of 1 Hz, which led to a significant increase in the test duration (especially for near-threshold crack growth rates). Thus, we may indicate only a trend to deceleration of the threshold crack growth of 17H1S steel in the corrosive environment (Fig. 3).

a b Fig. 3. Fatigue crack growth curves of 17H1S steel in the as-received state (a) and after long term operation (b) in air and NS4 solution.