PSI - Issue 39

Jesús Toribio et al. / Procedia Structural Integrity 39 (2022) 488–493 Author name / Procedia Structural Integrity 00 (2021) 000–000

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4. Micro-crack paths Schematically, the micro-fracture behaviour ( micro-crack path distribution) of the samples can be described in four zones, cf. Fig. 5: (i) in all cases fracture initiates at the periphery of the net section in a small region with ductile appearance, the so-called tearing topography surface (TTS), a term coined by Thompson and Chesnutt (1979) and Costa and Thompson (1982), and linked to hydrogen-assisted micro-damage (HAMD) in pearlitic steel following the research by Toribio et al. (1991, 1992), Toribio and Vasseur (1997) and Toribio (1997, 2012); (ii) after this, fracture continues by a sort of special micro-void coalescence MVC* (or quasi -MVC), a special MVC consisting of a mixture between classical MVC and TTS; (iii) later, fracture progresses in an intermediate zone formed by classical MVC; (iv) finally, the external crown appears in the whole fracture surface ( micro-fracture map MFM). It is important to emphasize that the HAMD in the form of TTS is a very special kind of TTS fractographic mode. A sort of stepped TTS topography (Fig. 6) whose appearance is very rough and irregular, showing evidence of micro crack deflections, branchings and bifurcations, a signal of certain longitudinal splitting or delamination, producing axial micro-cracking in the cold drawing direction (wire axis).

Fig. 6. Stepped TTS topography.

5. Discussion In the matter of the physical crack path in each of the described four microscopic fracture topography zones ( crack propagation sub-paths ) of the full MFM and of the fracture profile ( macro-crack path shown in Fig. 5), the crack propagation angle, measured in relation to the transverse (mode I) propagation path, defines the degree of mode mixity and thus the level of anisotropic hydrogen embrittlement behaviour . With regard to this, the order of crack propagation angles is as follows: α 3 < α 2 < α 1 < α 4 . Apart from the stepped appearance (shown in Fig. 6) of the TTS mode in the first propagation region 1 (cf. Fig. 5) resembling axial micro-cracking, the third propagation zone or intermediate region (cf. Fig. 5) also exhibits clear evidence of axial micro-cracking, as shown in Figs. 3a and 4a, indicating local micro-crack deflections representing embryos of anisotropic fracture . Local micro-deflections are induced by the oriented pearlitic microstructure after manufacturing by cold drawing (Toribio and Ovejero, 1997; 1998a; 1998b; 1998c). The embryos do not transform into real macro-crack deflections due to the low level of stress triaxiality (constraint) in the smooth wires. Toribio and Ayaso (2020) showed that both triaxiality and microstructural orientation are needed to produce fracture path deflection in cold drawn pearlitic steel. Axial micro-cracking also appears in standard tension tests performed in air (Toribio et al., 2016). 6. Conclusions Cold drawn pearlitic steel wires (commercial prestressing steel wires) exhibit axial micro-cracking paths and locally anisotropic hydrogen embrittlement behaviour, even for low levels of stress triaxiality in the absence of stress concentrators such as cracks or notches.