PSI - Issue 59

Jesús Toribio et al. / Procedia Structural Integrity 59 (2024) 24–30 Jesús Toribio / Procedia Structural Integrity 00 ( 2024) 000 – 000

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This paper studies the notch tip strain rate (NTSR), local strain rate at a notch tip or local strain rate in the vicinity of a notch tip. It is the really governing variable in EAF processes involving notched specimens. 2. Kinematic effects in environmentally assisted fracture (EAF) A well known technique for the evaluation of EAF processes is the performance of slow strain rate tests (SSRT) on initially smooth, pre-cracked or notched specimens, in which a constant displacement rate is externally applied on the sample ends up to final fracture. The advantages of such a technique have been outlined in previous works by Kim and Wilde (1979), Parkins (1979) and Scully (1979), and they become even more important when compared to those of constant load or constant strain tests (Puiggali et al., 1987). The primary conceptual advantage of the SSRT technique is the use of the strain rate as the main test variable, which allows an analysis of the very relevant transient processes in EAF phenomena. Constant load and constant strain tests represent, thus, limit cases of SSRT when the strain rate tends to zero and does not play the role of a test variable. Final fracture is always reached in SSRT more rapidly than in the constant load or constant strain tests, since in the former the environmental process is accelerated by imposing an increasing external load up to final fracture. A higher stress level is thus applied in SSRT in a more rapid and aggressive manner. The SSRT technique provides other relevant benefits as the realistic approach to service failures (Parkins, 1979), the establishment of an electro-chemically dependent range of strain rates within which EAF occurs (Parkins, 1979), the rapid identification of environment/metal combinations able to produce EAF (Kim and Wilde, 1979), and the quantitative rankings of EAF properties of metals and alloys having similar microstructures (Kim and Wilde, 1979). 3. The use of cracked and notched specimens in slow strain rate testing (SSRT) The use of cracked or notched specimens favours the localization of the environmental attack just at the crack or notch tip, thus decreasing the experimental scatter. Cracked samples can be prepared by fatigue, whereas notched samples require previous machining, more expensive in general. That is one of the reasons of the fact that cracked specimen testing has become more widespread in the last years. However, the use of notched samples has been strongly recommended more recently for experimental research on EAF using SSRT, in particular for fundamental studies of hydrogen embrittlement of metals (Thompson, 1985; Wang et al., 2005a, 2005b, 2007; Ayas et al., 2014). Furthermore, the triaxial stress state created in the vicinity of the notch has a synergistic effect combined with the environmental action, since it accelerates the hydrogen diffusion towards the points of maximum hydrostatic stress, on the basis of a mechanism of hydrogen transport in metals and alloys by stress-assisted diffusion, as discussed by Toribio and Kharin (1997a, 1997b, 1997c, 1998, 2000), Toribio et al. (2010) and Toribio and Kharin (2015). EAF in the presence of cracks and notches is a local phenomenon. The effect of the environment is localized at the crack or notch tip – the place where fracture initiates – or its close vicinity, so that local variables (stress, strain and strain energy density) must be relevant in such a process. In addition, cracking, damage and fracture are clearly time-dependent phenomena and, consequently, local kinematic variables – more precisely the local strain rate at the crack or notch tip – should play a very important role. The importance of localized transient processes in fracture under aggressive environment has been pointed out previously. It is generally accepted that rupture of oxide film, passivation, or hydrogen diffusion, are rate determining steps in environmental cracking (Scully, 1980). Regarding the effect of strain rate, a great research effort has been made in the past (Ford and Silverman, 1980; Hinton and Procter, 1983; Herbsleb and Schwenk, 1985; Mayville et al., 1987, 1989; Burnell et al., 1987; Magnin et al., 1990) which has demonstrated that failure load under aggressive environment is a function of the externally applied displacement rate. For corrosion-assisted phenomena, the balance between oxide film rupture and growth of the passivation film makes the dependence non monotonic (Kim and Wilde, 1979): when the displacement rate is very slow (in the limit constant load test) the sample becomes passivated, and when this rate is very fast, the dissolution does not have time to progress. Between both limits a minimum value of the fracture load is reached. For hydrogen embrittlement processes, fracture load is a monotonic increasing function of the displacement rate (Kim and Wilde, 1979, Burnell et al., 1987): the higher the displacement rate, the shorter the time for hydrogen entry and diffusion into the metallic specimen.