PSI - Issue 16

Ihor Dmytrakh et al. / Procedia Structural Integrity 16 (2019) 113–120 Ihor Dmytrakh et al. / Structural Integrity Procedia 00 (2018) 000 – 000

119

7

fatigue fracture of steel. As the hydrogen concentration ( C H = 0.514 and 1.231 ppm) increases further, the topography of the fracture surfaces becomes more inhomogeneous, and we observe the appearance of traces of breaking and quasicleavage, i.e., the mechanism of fatigue crack growth again becomes gradient-mixed (Figs. 7c and d). In the course of the digital processing of images of the fracture surfaces, we detected the domains of realization of the shear mechanism of fatigue crack growth. Every image has an area of 786,432 pixels or S = 42.219 µm 2 . By using a special procedure developed by Kosarevych et al. (2013), the authors revealed the areas with shear mechanism of fatigue crack growth. These areas are marked by rectangles (Fig. 7). It was discovered that their area in Fig. 7b is greater than in Figs. 7а, c, and d, which corresponds to the predomin ance of this mechanism of crack growth at C H = 0.209 ppm. For the final representation of the accumulated results, we used the parameter k equal to the relative fraction of the image area (in %), where the shear mechanism of fatigue crack growth is realized. The dependence of the parameter k on the concentration C H (Fig. 8) has a clear maximum at C H ≅ 0.209 ppm, which additionally confirms our conclusions concerning the ambiguity of the influence of the volume concentration of hydrogen on the fatigue crack growth in steel.

Fig. 8. Dependence of the characteristic parameter of fracture surface k on the hydrogen concentrations C H in the specimens ( 20 MPa m  ΔK ).

The accumulated results and their analysis enable us to formulate the following assertion: there exists a value of hydrogen volume concentration in the material C H = C H * for which the resistance to crack propagation increases, and the diagram of cyclic crack-growth resistance shifts to the region of higher values of SIF. For the analyzed steel C H * = 0.209 ppm. It should be emphasized that this value is close to the characteristic value C H * = 0.227 ppm, for which the mechanism of influence of hydrogen on the deformation of steel under quasistatic loads changes. Hence, the indicated characteristic (threshold) values of hydrogen concentration in the material corresponding to the transition from plasticization to embrittlement can be regarded as key parameters for the evaluation of the serviceability of structural materials in hydrogen-containing working media. We reveal basic regularities of the influence of hydrogen concentration on the fracture strength of ferritic- pearlitic low-alloyed pipe steels. In particular, we establish the characteristic value of hydrogen concentration for which the mechanism of its influence under the conditions of uniaxial quasistatic tension of the material changes, namely, below this value, hydrogen enhances the plasticity of the material, while above this value, it promotes its embrittlement. For the first time, we discovered an ambiguous relationship between the fatigue crack growth rate and the volume concentration of hydrogen under the conditions of cyclic loading of low alloyed steels in hydrogen-containing media and established the existence of a certain value of its volume concentration for which the cyclic crack-growth resistance of steel increases. The main outcome of the analysis of the accumulated results is the fact that, for low volume concentrations of hydrogen in the metal ( C H = 0.2 – 0.3 ppm), the process of plastic deformation of low-alloy steels is facilitated. This phenomenon is observed in different tests (Fig. 9): determination of the yield point σ y of cylindrical specimens (curve 1); finding the fracture energy U f of specimens with stress concentrators (curve 2), as it was made by Capelle et al. (2011), and evaluation of the fatigue crack growth rate da/dN (curve 3). In other words, the influence of 4. Concluding remarks