PSI - Issue 16

Ihor Dmytrakh et al. / Procedia Structural Integrity 16 (2019) 113–120 Ihor Dmytrakh et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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Fig. 5. Dependences of the fatigue crack growth rate da / dN at 20 MPa m  ΔK on the hydrogen concentration C H in the material: (1) test series A, (2) test series B.

Fig. 6. Dependences of the range Δ K on the hydrogen concentration C H for da / dN = 10 − 4 mm/cycle: (1) test series A, (2) test series B.

We analyzed the specific features of the fracture surface for a bounded domain corresponding to 20 MPa m  ΔK , i.e., under the conditions where the influence of concentration C H on the crack growth rate is especially pronounced (Figs. 5 and 6). By the visual observation of images, it was discovered that the fracture surfaces were modified depending on the parameter C H (Fig. 7).

Fig. 7. Specific features of the fracture surfaces of the specimens during fatigue crack growth at 20 MPa m  ΔK depending on the hydrogen concentration C H (× 500): (a) C H = 0.001 ppm, (b) 0.209, (c) 0.514, and (d) 1.231 ppm. The domains of realization of the shear mechanism of fatigue crack growth are marked by rectangles. If the hydrogen concentration in specimens is very low ( C H = 0.001 ppm), then the surface topography is inhomogeneous, which corresponds to the mixed mechanism of fatigue crack growth: tension and shear (Fig. 7а). In this case, we also detect a typical topography of brittle quasicleavage. As the hydrogen concentration increases ( C H = 0.209 ppm), the surface topography becomes more homogeneous as a result of growth of the area where the shear mechanism of fatigue crack growth is realized (Fig. 7b). This reveals the growth of plastic strains in the process of