PSI - Issue 59

Jesús Toribio et al. / Procedia Structural Integrity 59 (2024) 145–150 Jesús Toribio / Procedia Structural Integrity 00 ( 2024) 000 – 000

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1. Introduction Hydrogen embrittlement (HE) is a general phenomenon of degradation appearing as a decrease of tensile strength in both smooth and notched specimens. Other names such as hydrogen damage or hydrogen degradation appear as adequate to describe the process. 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 (Wang 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 (Wang et al., 2007; Ayas et el., 2014). In the matter of HE of pearlitic steel, Toribio (1992, 1993, 1996) showed the key role of fractography to analyze hydrogen-assisted micro-damage (HAMD). It was proven that hydrogen diffusion predominates over dislocation transport (Toribio, 1992, 1996) driven by the hydrostatic stress gradient (Toribio, 1993; Toribio and Ayaso, 2004). This paper studies the effect of notch geometry (and particularly notch depth) on the distribution of hydrogen concentration, for the limit case of the steady-state regime, in circumferentially notched specimens of pearlitic steel containing both sharp and blunt notches subjected to different values of remote axial stress. 2. Numerical modelling The finite element method (FEM) was used to study how the presence of the notch affects hydrogen diffusion. The geometry was a round bar with 11.25 mm of diameter D and 50 mm of length with a circumferentially-shaped notch (of radius R and depth C ) under axial loading. The geometric parameters were: C / D = {0.1, 0.2, 0.3 and 0.4}, R / D = 0.04 (sharp notches) and R / D = 0.40 (blunt notches), cf. Fig. 1. The mesh is more refined in the area next to the notch and isoparametric elements of four nodes have been used. The material studied was a high strength steel with Young’s modulus E = 200 GPa, yield strength  Y = 600 MPa, UTS  R = 1200 MPa and Poisson’s ratio ν = 0.3. The computations used J2-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 wer e the following: diffusion coefficient D = 6.6·10 -11 m 2 /s (Lillard et al., 2000) and the molar partial volume of hydrogen in the steel V H = 2·10 -6 m 3 /mol (Wagenblast and Wriedt, 1971).

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C

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D/2

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( b ) ( c ) Fig. 1. Notched geometries: (a) sharp notches; (b) blunt notches; (c) characteristic parameters.