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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4

where c 0 is the equilibrium hydrogen concentration in the metal in the absence of any stresses. This equation is also the stationary solution of the diffusion problem. 5. Results and discussion Fig. 3 plots the boundary ( notch-tip ) concentration of hydrogen c Г / c o represented as a function of the load ratio F / F max . Results show how such a boundary value increases clearly with the remote stress (externally applied load) and slightly decreases with the notch depth, i.e., the equilibrium boundary value of hydrogen concentration is slightly higher for shallow notches (both sharp and blunt). With regard to the implications of the present numerical analysis in experimental aspects of HE and notch tensile strength of pearlitic steel, previous research on the topic showed evidence that the fracture process starts in the vicinity of the notch tip, inside a critical zone whose fractographic appearance resembles a sort of microplastic tearing: the so-called tearing topography surface (TTS), as described by Toribio et al. (1991), Toribio and Vasseur (1997) Toribio (1997), Toribio (2012) and Toribio (2018).

2.0

2.0

1.5

1.5

1.0

1.0

R/D=0.04

0.0 0.2 0.4 0.6 0.8 1.0 C/D=0.1 C/D=0.2 C/D=0.3 C/D=0.4 F/F max R/D=0.40

 0

 0

c /c

c /c

0.0 0.2 0.4 0.6 0.8 1.0 C/D=0.1 C/D=0.2 C/D=0.3 C/D=0.4

0.5

0.5

0.0

0.0

F/F

max

(a)

(b)

Fig. 3. Dimensionless hydrogen concentration at the boundary ( notch tip ) c Г / c o vs load ratio F / F max for sharp notches (R/D = 0.04; left) and blunt notches (R/D = 0.40; right).

Toribio et al. (1991) proved that the hydrostatic stress is the macroscopic variable governing the microscopic fracture of pearlitic steels in hydrogen, i.e., governing mainly the hydrogen penetration at the inner boundary of the notched specimens and further transport towards the inner points of the specimens by a mechanism of transport by stress-assisted diffusion of hydrogen (Toribio 1992, 1993, 1996). Considering these evidences, the position of the maximum hydrostatic stress point towards which hydrogen mainly diffuses is of the highest interest in hydrogen embrittlement analyses. It is the point of maximum hydrogen concentration in the steady state regime. Fig. 4 shows the maximum hydrostatic stress point depth x S as an increasing function of the load ratio F / F max . As described by Toribio (1992, 1993), it is the point towards which hydrogen diffuses (the point of maximum hydrogen concentration in the steady state regime), so that is of the highest interest in HE. This important length has a relevant fractographic meaning, since it coincides with the asymptotic depth (for quasi-static tests or steady state regime) of the hydrogen-assisted micro-damage (HAMD) area ( tearing topography surface or TTS)) in pearlitic steels, as reported by Toribio (1992, 1993, 1996, 1997, 2012 and 2018), Toribio et al. (1991) and Toribio and Vasseur (1997).