PSI - Issue 41

Jesús Toribio et al. / Procedia Structural Integrity 41 (2022) 724–727 Jesús Toribio / Procedia Structural Integrity 00 (2022) 000–000

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• The microscopic appearance of the TTS region (micro-fracture mode) resembles micro-damage, micro-cracking or micro-tearing due to hydrogen degradation at the finest micro-mechanical level. • The TTS mode is a slow (or subcritical) crack growth topography associated with the stage II or plateau in the crack growth kinetics curve, i.e., with a constant crack growth rate independent of the stress intensity factor. • The TTS mode appears only under cathodic potentials (associated with hydrogen environment), but not under anodic ones (associated with anodic dissolution), thereby providing fractographic evidence of hydrogen effects. • The TTS size increases when the electro-chemical potential becomes more cathodic or negative. This is a consequence of the higher amount of hydrogen at the specimen surface for the more cathodic potentials. • The TTS size slightly increases as the pH decreases (becomes more acid), since the amount of hydrogen supplied to the sample is higher for lower pH values. • The aforesaid effect is really slight as a consequence of the well known local acidification in the vicinity of the crack tip, independently of the bulk pH value. • The TTS size clearly decreases as the fatigue load increases in cracked specimens, due to the presence of compressive residual stresses near the crack tip induced during fatigue precracking. • There is an increasing trend of the TTS size as the time to failure increases (or the applied strain rate decreases), a consequence of a mechanism of transport based on stress-assisted hydrogen diffusion and time derivatives. • The TTS size is clearly influenced by the geometry of the sample, through the hydrostatic stress distribution in its vicinity, consistent with a hydrogen transport mechanism by stress-assisted diffusion. • The asymptotic depth of the TTS zone in quasi-static tests ( steady-state situation) reaches that of the maximum hydrostatic stress point in the sample, thereby confirming that hydrogen diffuses towards such a point. 4. Hydrogen embrittlement (HE) tests To evaluate the HE behaviour of high-strength pearlitic steel wires, constant strain tests (CST) were carried out on three-point bend prismatic pre-cracked specimens, as described by Toribio and Lancha (1998). All tests were carried out under potentiostatic control, and the electrochemical potential was E = –1200 mV SCE. Both the load on the sample and the crack length were measured during the test, and strain was increased step by step, to obtain the crack growth kinetics (CGK) curve da/dt-K I , plotted in Fig. 2. The threshold stress intensity factor is K th = 0.35 K 1C . For higher levels of K I there is a plateau (with da/dt  1.5x10 –7 m/s), then a sudden increase up to about 10 –3 m/s, a slightly sloped section and finally the failure. The actual plateau corresponds to a real sub-critical stage of type II (with sub-critical crack growth rate) followed by a second pseudo-plateau (stage III) in which the crack growth rate can be considered as critical (near failure). While the first plateau may be assumed to be environmentally (HE) governed and sub-critical, the second pseudo-plateau has a more defined mechanical nature and critical (near-failure) character, although a combined effect of hydrogen presence and stress concentration could exist. However, given the elevated crack growth rate, it is more mechanical failure than corrosion-related process. An important question is to elucidate the meaning of the characteristic stress intensity levels in the crack growth kinetics curve (Fig. 2). It is seen that a critical stress intensity factor does exist as K H = 0.55 K 1C for which there is a sudden increase of the crack growth rate accompanied by a change of the microscopic mode of fracture from TTS to cleavage (just after the stage II or plateau in the crack growth kinetics curve). 5. Conclusion The fracture parameter K H is seen to be the transition stress intensity factor in the crack growth kinetics curve (da/dt-K I ) at which the crack growth rate progresses from the subcritical to the critical regime and the microscopic mode of fracture changes from the tearing topography surface (TTS) mode to cleavage-like propagation.