PSI - Issue 59

Jesús Toribio et al. / Procedia Structural Integrity 59 (2024) 31–35 Jesús Toribio / Procedia Structural Integrity 00 ( 2024) 000 – 000

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This paper studies the crack tip strain rate (CTSR), local strain rate at a crack tip or local strain rate in the vicinity of a crack tip. It is the really governing variable in EAC processes involving cracked specimens. 2. Kinematic effects in stress corrosion cracking (SCC) Stress corrosion cracking is a physical phenomenon that always involves a time-dependent process, either dissolution of material produced by the environment, or embrittlement due to hydrogen created by chemical reactions. In every case, there is a balance between the action of the aggressive environment and the time, which determines the severity of the process. The importance of kinematic variables and of corrosion-deformation interactions in SCC processes has been pointed out in basic references of the topic (Scully, 1980). As a consequence, stress corrosion phenomena depend markedly on kinematic testing variables such as the rate of load, displacement or strain, as discussed in detail in previous research papers by Ford and Silverman (1980), Hinton and Procter (1983), Herbsleb and Schwenk (1985) and Mayville, et al. (1987, 1989). However, the displacement rate is not the variable actually representative and it allows the establishment of 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 one needs 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). 3. Previous attempts to evaluate the crack tip strain rate (CTSR) Many difficulties arise in determining the strain distribution and the spreading of the plastic zone in the close vicinity of a crack tip (Patel and Jarman, 1985). Lidbury (1983) states that the crack-tip (or effective) strain rate under conditions of monotonic loading and general yielding can be 10 to 20 times the nominal or applied strain rate. Papers by Maiya (1987) and Maiya and Shack (1985) provide a definition of the crack tip strain rate associated with the J -integral. In the article by Congleton et al. (1985) an estimation of the strain rate at a crack tip is made 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 the kinetics of SCC. Finally, and emphasizing the difficulty of a correct determination of the local strain rate at a crack tip, Andresen and Ford (1988) propose a simply empirical value of the crack tip strain rate. 4. Shortcomings of previous models to evaluate the crack tip strain rate (CTSR) An inherent limitation of all these expressions for the local strain rate at the crack tip is that they do not take into account the constitutive equation of the material, whose incidence in the local strain rate is not negligible, as is demonstrated by Toribio (1997a, 1997b) for cracks and by Toribio (1998) and Toribio and Elices (1992) for notches. The main consequence of that oversimplification is the prediction of a constant local strain rate at the crack tip if the typical condition of constant extension rate is achieved during a SCC test. Another important fault of previous models is the lack of proper definition of the reference length for evaluating the crack tip strain rate (CTSR). This issue is properly solved in the new attempts to evaluate the local strain rate at a crack or notch tip or its vicinity ( crack tip strain rate CTSR and notch tip strain rate NTSR) with a quite rigorous definition of the global and local reference lengths to calculate the global and local strains, as described in previous research by the author, cf. Toribio and Elices (1992), Toribio (1997a), Toribio (1997b), Toribio (1998).