PSI - Issue 41

Jesús Toribio et al. / Procedia Structural Integrity 41 (2022) 736–743 Jesús Toribio / Procedia Structural Integrity 00 (2022) 000–000

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From the finite element results, it is possible to calculate the von Mises effective or equivalent stress  eff c in the critical situation (i.e., at the end of the SSRT). It is useful to express the experimental results in terms of the following dimensionless variable to express the embrittlement effect:  =  eff c /  eff o (6) where  eff o is the critical (fracture situation) von Mises effective or equivalent stress in the air test. To represent the strain rate effect, the following dimensionless variable can be defined:  = << d  L /dt >> = << d  NT /dt >> x s 2 D*  (7) where << d  L /dt >> = << d  NT /dt >> is the space-time average of the local strain rate in the vicinity of the notch tip or NTSR (space average over the hydrogen affected region; time average over the tests duration), x s the depth of the point of maximum hydrostatic stress (towards which hydrogen diffuses), and D* the hydrogen diffusion coefficient. The TTS zone was considered as the critical region (of depth x TTS ), and the following values were used: The key role of the maximum hydrostatic point (located at a depth x S from the notch tip) is due to the fact that the triaxial stress state created in the vicinity of the notch tip accelerates the hydrogen diffusion towards the points of maximum hydrostatic stress, on the basis of a mechanism of hydrogen transport in metals by stress-assisted diffusion, as discussed extensively by Toribio and Kharin (1997a, 1997b, 1997c, 1998, 2000, 2015). Calculating the variables  (6) and  (7) for all SSRT, the plot of Fig. 5 is obtained, showing the test results in dimensionless form, as a function of the NTSR, averaged over the test period and the hydrogen affected area. Results expressed in this manner have an objective character, because most results for all geometries fit in the same curve, thereby demonstrating the role of the local strain rate in the vicinity of the notch tip (i.e., the NTSR) in the hydrogen embrittlement of notched samples. This common curve represents the functional relationship between the critical equivalent stress of the material in hydrogen and the NTSR, and therefore is a kinematic formulation of the fracture criterion in aggressive environment, i.e., the critical parameter of the material as a function of the NTSR. Fig. 5 also shows that there are two asymptotic situations: the ultra-fast tests (very high local strain rate) and the quasi-static tests (extremely low local strain rate). The first is fast enough to avoid hydrogen diffusion towards the inner points of the sample, whereas the second is slow enough to allow the stationary condition for the hydrogen diffusion problem to be reached (where x TTS = x S ). In the ultra-fast tests (horizontal full line in the right part of Fig. 5) the critical equivalent stress in hydrogen environment reaches an upper limit: 93% of its value in inert environment. This loss of load bearing capacity with respect to the inert reference environment is a consequence of the quasi-instantaneous absortion of the hydrogen adsorbed on the sample surface. In the quasi-static tests (horizontal dashed lines in the left part of Fig. 5) the critical equivalent stress in hydrogen environment reaches an asymptotic value different for each geometry. This value limits the range of applicability of the kinematic formulation of the fracture criterion: for local strain rates higher than those for the quasi-static tests, the critical equivalent stress of the material in hydrogen is a universal function (geometry-independent) of the local strain rate; for NTSR lower than the quasi-static ones, the critical equivalent stress of the material is constant for each geometry (a constant for each stress triaxiality). In the latter case, the strain rate is so low that the equilibrium concentration hydrogen-metal is reached during the test, and therefore the embrittlement does not depend on the strain rate, although it is clearly influenced by the geometry through the stress state near the notch tip.  eff c = 1260 MPa x S (A) = 0.30 mm x S (B) = 1.20 mm x S (C) = 0.80 mm x S (D) = 1.20 mm D* = 5x10 –11 m 2 /s