PSI - Issue 28

Jesús Toribio et al. / Procedia Structural Integrity 28 (2020) 2444–2449 Jesús Toribio et al. / Procedia Structural Integrity 00 (2020) 000–000

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Increasing the stress concentration (notch severity) makes hydrogen more effective as an embrittling agent, but from a certain value of stress concentration a relatively stable status is reached (Hardie and Liu, 1996) or even this phenomenon (embrittlement) decreases for low-alloy steels (Walter and Chandler, 1971). The role of notches in the hydrogen embrittlement of high-strength steel has been studied in the past from the research works by Toribio and Elices (1992) and Wang et al. (2005a, 2005b, 2007) to the more recent article by Ayas et al. (2014), analyzing the loading rate effects in terms of the local strain rate (Toribio and Elices, 1992) or just the global crosshead speed (Wand et al., 2005a), studying the geometry effects in terms of stress triaxiality (Toribio and Elices, 1992) or stress concentration factor (Wang et al., 2005b, 2007) and formulating fracture criteria for notched samples of high strength steels in the presence of hydrogen in terms of the distortional component of the strain energy density (Toribio and Elices, 1992) or some local stress (Wand et al., 2007; Ayas et el., 2014). In the matter of hydrogen embrittlement of pearlitic steel, previous research works (Toribio 1992, 1993, 1996) established the importance of fractographic analysis to elucidate the physical mechanisms of microdamage and the main hydrogen transport mode in the material. On the basis of plasticity effect in pearlite surrounded by hydrogen, it was seen that diffusion is predominant over dislocational dragging (Toribio, 1992, 1996) it being driven by the gradient of hydrostatic stress (Toribio, 1993). The numerical analysis of the hydrostatic stress fields in notched geometries demonstrates that sharp-notch specimens are the most adequate for hydrogen embrittlement testing and evaluation, using either shallow notches to detect surface effects or deep notches to analyze hydrogen penetration, as studied by Toribio and Ayaso (2004). This article studies the effect of sharp notch depth on the distribution of hydrogen concentration, for the limit case of the steady-state regime, in circumferentially notched specimens of pearlitic steel subjected to different values of remote axial stress. 2. Numerical modelling Modelling was performed using the finite element method FEM (with MSC Marc software) in order to analyze the effect of the presence of a stress concentrator (notch) on hydrogen diffusion. The studied geometry was a circumferentially notched round bar specimen with 11.25 mm of external diameter D and 50 mm of length, subjected to axial load at its ends. The notch was characterized by its radius R and its depth C . For the calculations, the notch geometries selected were those corresponding to the following parameters: C / D = {0.1, 0.2, 0.3 and 0.4} and R / D = 0.04 corresponding to a sharply notched sample (Fig. 1a). Since the problem has symmetry of revolution, it was reduced to a two-dimensional (2D) study with the corresponding boundary conditions (Fig. 1b). The mesh is more refined in the area next to the notch and isoparametric elements of four nodes have been used (Fig. 1c). The material studied was a commercial medium-high strength steel of pearlitic microstructure whose characteristic stress-strain curve σ - ε (obtained through the standard tensile test) is shown in Fig. 2. The Young’s modulus E of the steel was 200 GPa and the Poisson’s ratio ν was 0.3. For the calculations it was considered plasticity with isotropic hardening, Von Mises yield criterion and large deformation updated Lagrange analysis. A temperature of 23ºC was considered. The properties of pearlitic steel relative to the diffusion of hydrogen were the following: diffusion coefficient D = 6.6ꞏ10 -11 m 2 /s (Lillard et al., 2000) and the molar partial volume of hydrogen in the steel V H = 2ꞏ10 -6 m 3 /mol (Wagenblast and Wriedt, 1971). 3. Mechanical analysis The FEM analysis allows a determination of stress-strain distributions in the samples for increasing levels of externally applied load (remote stress) on the specimens. On the basis of this mechanical analysis , the distribution of hydrogen concentration will be obtained ( chemical analysis ). The load-displacement curves F - u are shown in Fig. 3 up to the maximum load F max (after this value the load decreases). The maximum displacement u max is greater for shallow notches than for deep ones, and there is a sort of similarity between the curves corresponding to different notch depth.