PSI - Issue 59

Jesús Toribio et al. / Procedia Structural Integrity 59 (2024) 104–111 Jesús Toribio / Procedia Structural Integrity 00 ( 2024) 000 – 000

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Fig. 2. Environmentally-assisted micro-damage region at the crack tip: (a) anodic regime of LAD ( – 400 mV vs. SCE) showing a damaged area ahead of the crack tip; (b) cathodic regime of HAC ( – 1200 mV vs. SCE) exhibiting tearing topography surface (TTS) ahead of the crack tip.

Fig. 3. TTS depth as a function of K max .

These K max -effects are produced by the presence of compressive residual stresses in the vicinity of the crack tip as a consequence of the cyclic plastic zone created by the fatigue pre-cracking procedure (cyclic residual stresses). The crack tip is — in a certain sense — prestressed by fatigue: the higher the overload level, the more pronounced the prestressing effect which delays the environmental damage process (metal dissolution or hydrogen entry) and improves the material performance in a corrosive environment. The next section models the cyclic residual stresses in the vicinity of a crack tip after loading and unloading, in order to derive conclusions about hydrogen-plasticity interactions in the near-tip region (in the close vicinity of the crack tip) so as to elucidate the predominant hydrogen transport mechanism (either hydrogen diffusion or hydrogen transport by dislocations) in pearlitic steel. 3. Mechanical analysis: modelling of cyclic residual stress laws It would be useful to know the cyclic residual stress distribution ahead of the crack tip generated by strain compatibility in the near-tip area due to the loading/unloading process after fatigue pre-cracking of the specimens. There are three kinds of difficulty in solving this problem. Firstly, from the theoretical point of view, the difficulty of knowing the exact near-tip stress distribution after fatigue pre-cracking (and therefore prior to the SCC test), because the stress singularity is relaxed as the plastic zone spreads. In addition, the numerical approach to the problem (finite element method, boundary integral equation method) is really complex due to the stress concentration and to the loading/unloading process. Finally, the residual stress distribution after fatigue pre-cracking is only representative at the beginning of the SCC test, since it changes during it. From these considerations it seems that the formulation of a simple model is sufficient indication of the residual stress distribution ahead of the crack tip after fatigue pre-cracking of the specimens.