PSI - Issue 41

Jesús Toribio et al. / Procedia Structural Integrity 41 (2022) 728–735 Jesús Toribio / Procedia Structural Integrity 00 (2022) 000–000

729

2

1. Introduction Environmentally assisted cracking (EAC) is a phenomenon that always involves a time-dependent process, either dissolution of material produced by the external environment, the so-called localized anodic dissolution (LAD) or pure stress corrosion cracking (SCC), or hydrogen embrittlement (HE), also called hydrogen degradation (HD), hydrogen assisted fracture (HAF) or, when cracks appear, hydrogen assisted cracking (HAC). The relevant role of kinematic variables in SCC processes has been analyzed in previous scientific research (Scully, 1980; Ford and Silverman, 1980; Hinton and Procter, 1983), so that SCC phenomena strongly depend on kinematic testing variables such as the rate of load or displacement (Mayville, et al., 1987; 1989). However, the displacement rate is not the governing variable and it allows only qualitative phenomenological relations. The same applies to the rate of loading or strain, since the former is clearly a global variable , whereas the latter is commonly used in its global sense , i.e., the displacement applied by the testing machine (external, nominal or applied testing displacement rate, or crosshead speed) divided by a reference length. To obtain quantitative relations and objective results, it is necessary to know the local strain rate at the crack or notch tip or in its vicinity, because at that point or zone the environmental attack is localized, and therefore the properly called local strain rate at the crack tip or crack tip strain rate (CTSR) controls the SCC process, as discussed by Scully and Moran (1988) and Rieck et al. (1989). The local strain rate at the crack tip —and not the externally applied testing displacement rate— has to be compared with the dissolution (or film rupture) and passivation rates, or with the hydrogen diffusion rate, depending on the considered process. This paper deals with the estimation of the CTSR, with particular emphasis on formulating a kinematic fracture criterion to describe the strain-rate effect in slow strain rate tests (SSRT) with precracked samples subjected to environmental conditions promoting hydrogen embrittlement. The criterion is formulated by expressing the critical fracture parameter of the material as a function of the CTSR governing the hydrogen entry. 2. On the crack tip strain rate (CTSR) Previous attempts exist in the scientific literature to calculate (or, at least, to estimate) the CTSR. Lidbury (1983) estimates the CTSR under conditions of monotonic loading and general yielding as 10 to 20 times the nominal or applied strain rate. Maiya (1987) and Maiya and Shack (1985) link the CTSR with the J -integral. Congleton et al. (1985) estimate the CTSR for an ideally plastic solid under plane strain and fully plastic conditions (Rice and Sorensen, 1978) and it was used by Parkins (1987, 1989, 1990) to study SCC kinetics. Andresen and Ford (1988) propose an empirical value of the CTSR. The inherent limitations of the described models to estimate the CTSR are two-fold. Firstly, they do not take into account material factors (constitutive equation of the material), whose incidence in the CTSR is not negligible. Secondly, there is a lack of proper definition of the reference length for evaluating the CTSR. These problems were addressed and solved by Toribio (1997a, 1997b) with regard to CTSR in cracked geometries and by Toribio (1998) and Toribio and Elices (1992) in the matter of notch tip strain rate (NTSR) in notched geometries. 3. Calculation of the crack tip strain rate (CTSR) In order to obtain the CTSR, the first step is to define the crack tip strain (CTS)  CT as that associated with the local reference length L CT (small enough to guarantee the convergence of the method, although greater than the material grain size) at the crack tip or its vicinity (Fig. 1):

∆L CT L CT

v CT z CT

 CT =

=





(1)