PSI - Issue 28

Jesús Toribio et al. / Procedia Structural Integrity 28 (2020) 2390–2395 Jesús Toribio et al. / Procedia Structural Integrity 00 (2020) 000–000

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Fig. 3. Scheme of the MFMs trend depending on the degree of cold drawing (Toribio and Vergara, 2013).

According to recent studies where the first three models of MFM were analyzed (Toribio and Vergara, 2013), the level of microstructural anisotropy induced by cold drawing in pearlitic steel wires contributes to generate a ring shaped MFM (model type III) due to a barrier effect and a more uniform circumferential distribution of material characteristics. In addition, it was demonstrated that, although the notch radius enhance the circumferential distribution of FPZ (ring-shaped TTS zone), the notch depth is the factor having a stronger influence on MFMs trend followed with cold drawing process. Hydrogen diffusion assisted by the transient and non-uniform stress field (CERTs) is driven by two parameters: the gradient of  C and the gradient of hydrostatic stress  (Krom et al., 1999). The later plays a relevant role in the matter of establishing the four models presented in this paper, since the distribution of hydrostatic stress  notably varies from one to another type of notch, so that the maximum value of  sometimes is achieved at the specimen surface (notch tip) and other times at the sample center (Toribio et al., 2011). The loading rate also plays a key role in the formation of the FPZ because hydrogen needs time to diffuse and reach the prospective fracture places. Accordingly, in the case of blunt notch type D, in which the maximum hydrostatic stress  is reached in the centre of the specimen (Toribio et al., 2011), the FPZ appears in the inner region of the cross sectional area of the sample (model IV) for long time of exposure to hydrogen, i.e., for low strain rate, type 1 (Table 1). 5. Conclusions The fracture surfaces in notched samples of pearlitic steel subjected to constant extension rate tests (CERTs) in an environment promoting hydrogen embrittlement (HE) may be classified into four schematic micro-fracture maps (MFMs). In all MFMs the fracture process zone (FPZ) is associated either with tearing topography surface (TTS) or with an area resembling micro-void coalescence (MVC* or quasi-MVC), i.e., a candidate to TTS that is not fully hydrogenated. The level of microstructural orientation (material anisotropy) induced by cold drawing in pearlitic steel wires contributes to generate a ring-shaped MFM due to a barrier effect and a more uniform circumferential distribution of material characteristics. The MFM associated with model IV, according to which the FPZ is formed in the centre of the specimen, appears only in the case of bluntly notched geometries (type D) tested at very low strain rates, due to the time needed for hydrogen to diffuse towards the fracture sites.