PSI - Issue 37

Jesús Toribio et al. / Procedia Structural Integrity 37 (2022) 1001–1006 Jesús Toribio / Procedia Structural Integrity 00 (2021) 000 – 000

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The MFM of Fig. 3 appears in all specimens tested between −1100 y −1400 mV versus SCE ( cathodic potentials ) for both strain rates. In the case of anodic potentials, fracture takes place outside the corrosion cell, so that only the cathodic ones (hydrogen embrittlement environmental conditions) will be analyzed in this paper. In all cases fracture initiates at the periphery of the net section (external surface of the wire), in a small region with ductile appearance (Fig. 4): the so-called tearing topography surface (TTS), a term coined by the researchers Thompson and Chesnutt (1979) and Costa and Thompson (1982). It has been proved by Toribio et al. (1991, 1992), Toribio and Vasseur (1997) and Toribio (1997, 2012) that the TTS micro-fracture mode is the fracture region associated with hydrogen-assisted micro-damage (HAMD) in pearlitic microstructures, whose shape and size are determined by the mechanical and environmental variables governing the hydrogen embrittlement process itself (stress triaxiality, strain rate, etc.). In this paper the TTS zone is assumed to be semi-elliptical with axes a and b (cf. Fig. 3). Next to the TTS zone, the fracture process zone (FPZ) appears (Fig. 5), formed by a sort of special micro-void coalescence (MVC* or quasi MVC). In this particular MVC* zone the voids are greater than those appearing in a classical MVC area that can be found in the intermediate zone (IZ, whose fractographic appearance is given in Fig. 6) and in the external crown (EC, whose fractographic appearance is given in Fig. 7). 4. Discussion Hypothetically, the second zone of fracture by quasi-MVC (or MVC*) is a candidate to TTS not sufficiently hydrogenated. Therefore, when the hydrogenation level is enough, The TTS zone appears indicating HAMD. In close areas not so sufficiently hydrogenated, quasi-MVC can be found because of a lower level of hydrogen content. Fracture progresses in a ductile manner (not assisted by hydrogen) later on, covering the IZ and the EC. The IZ presents zones with axial micro-cracking in the form of local micro-crack deflections that represent embryos of anisotropic fracture . They are produced by the oriented pearlitic microstructure after manufacturing by cold drawing, as described by Toribio and Ovejero (1997; 1998a; 1998b; 1998c) but they do not transform into a real macro-crack deflection due to the lack of stress triaxiality of the smooth specimens. As a matter of fact, it has been proved by Toribio and Ayaso (2020) that there is a necessity of both traxiality and microstructural orientation to produce fracture path deflection in cold drawn pearlitic steel wires. This axial micro-cracking also arises in standard tension tests with no environmental assistance (Toribio et al., 2016), 5. Conclusions The sequence of fracture is: (i) firstly, HAMD takes place in the form of TTS; (ii) secondly, fracture continues in a sort of MVC* (or quasi-MVC); (iii) later, fracture progresses in the IZ; (iv) finally, the EC takes place.

Fig. 4. Tearing topography surface (TTS).