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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4.2. Interactions between crack-tip plasticity and hydrogen On the basis of previous definitions given in eqs. (1)-(6), Fig. 5 shows the plastic zone size during fatigue pre cracking (monotonic for K = K max and cyclic for K = K min = 0) and at the end of the SCC tests (LAD & HAC).

Fig. 5. Plastic zone size during fatigue pre-cracking (monotonic for K=K max and cyclic for K= K min ) and at the end of the LAD tests (left-hand side). Plastic zone size during fatigue pre-cracking (monotonic for K= K max and cyclic for K= K min ) and at the end of the HAC tests (right-hand side). In the latter, the TTS depth is also plotted as a decreasing function of the maximum SIF during fatigue pre-cracking K max . For LAD phenomena (Fig. 5; left) there is a strong protective effect caused by the fatigue pre-cracking load, and thus the final plastic zone during the LAD test surpasses clearly the cyclic and the monotonic plastic zones created during fatigue pre-cracking. The transition topography which appears in the microscopic fractographic analysis is probably created by the dissolution of the very strongly damaged zone close to the crack tip. In the case of HAC (Fig. 5; right) the protective effect is weaker. However, the compressive stresses generated by the cyclic plastic zone have a prolonged effect during the experiments, so as for K max = 0.80 K IC the monotonic plastic zone created by fatigue is not exceeded during the HAC test. With regard to the microscopical modes of fracture, the transition topography in the form of TTS is observed in all tests. As shown in Fig. 5 (right), its size is related to the difference between the sizes of the plastic zone at the end of the HAC test and the monotonic plastic zone created by the fatigue maximum load, which seems to indicate that such a microscopic mode of fracture – a subcritical mode associated with HAMD – develops after the previous tensile stress distribution is recovered. The afore-said fact is consistent with a mechanism of hydrogen transport by stress-assisted hydrogen diffusion according to which hydrogen diffuses not only to the minimum concentration points, but also towards the maximum hydrostatic stress locations. Thus for K max = 0.80 K IC the TTS size is negligible, the monotonic plastic zone created by fatigue is not exceeded during the HAC test and hydrogen penetration is impeded by this residual plastic zone Furthermore, for light fatigue pre-cracking regimes (K max < 0.50 K IC ; cf. Fig. 5, right), the TTS depth (i.e., the HAMD region associated with hydrogen effects and thus with hydrogen presence) clearly exceeds the plastic zone size (i.e., the only zone where dislocational movement can take place), so that the hydrogen transport must have been by lattice diffusion and not by dislocational dragging, since the movement of any dislocation is confined inside the plastic zone. This experimental evidence is fully consistent with similar ones (Toribio, 1992; 1996) obtained with hydrogen embrittlement of notched specimens in which, for certain tests, the TTS zone (HAMD associated with hydrogen effects) exceeds the plastic zone, the only domain in which dislocation’s movement takes place. 5. Conclusions 1. Residual stresses generated in the vicinity of the crack tip during fatigue pre-cracking of specimens have shown to be relevant in determining the material resistance to SCC. 4.3. Role of diffusion in the hydrogen transport in pearlitic steel