PSI - Issue 33

Jesús Toribio et al. / Procedia Structural Integrity 33 (2021) 1215–1218 Jesús Toribio / Procedia Structural Integrity 00 (2021) 000–000

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The hydrogen embrittlement phenomenon is strongly affected by the presence of notches (Toribio, 1992, 1993; Wang et al., 2007; Ayas et al. 2014). On the basis of fractographic evidence, it was seen in pearlitic steel that hydrogen transport by diffusion is predominant over dislocational dragging, the gradient of hydrostatic stress playing a relevant role (Toribio, 1992, 1993). The paper studies the effect of the notch depth on the distribution of hydrogen concentration, for the limit case of the steady-state regime, in round notched samples of steel subjected to different values of remote axial stress. 2. Numerical modelling Modelling was performed using the finite element method. The geometry was a round notched sample with 11.25 mm of diameter D and 50 mm of length under axial load (Fig. 1). The geometries were C / D = {0.1, 0.2, 0.3 and 0.4} and R / D = 0.40 (blunt notch; Fig. 1). The problem 2D (by symmetry) with the boundary conditions (Fig. 1b). The mesh is more refined in the area next to the notch and isoparametric elements of four nodes were used (Fig. 1c). The material studied was a medium-high strength steel whose characteristic stress-strain curve σ - ε 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. 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 were 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). 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.

R

C

C/D = 0.1

D/2

F

C/D = 0.4 (a)

(b) (c) Fig. 1. Two-dimensional (2D) geometry of the considered specimen: (a) bluntly-notched specimen with shallow (C/D = 0.1) and deep (C/D= 0.4) notch; (b) characteristic parameters; (c) finite element mesh.