PSI - Issue 28

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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. 1. Geometries of round notched samples used in the present study.

3. Fractography A fractographic analysis was performed on the broken samples by scanning electron microscopy (SEM), and different fracture topographies were found, namely: (i) microvoid coalescence (MVC) associated with material ductile behaviour; (ii) cleavage ( C ) linked to brittle behaviour; (iii) tearing topography surface (TTS), representing a characteristic fracture mode associated with HAF processes taking place in pearlitic steels (Toribio et al., 1992); (iv) quasi - MVC (MVC*), resembling MVC that could be considered as candidates to TTS zones in which hydrogenation is not enough (Toribio and Vasseur 1997; Gamboa and Atrens 2003). As sketched in Fig. 2, four types of micro-fracture map (MFM) were established from the fractographic analysis, according to the shape of the TTS region. Fig. 2a represents the MFM type I, with a fracture process zone (FPZ) constituted by a well-defined TTS area (with a more or less semielliptical shape) placed at the wire surface. This zone extends over a relatively narrow region (grey zone). Fracture initiates and grows subcritically there; afterwards, when the critical time is reached, it grows by cleavage ( C ) in divergent directions covering practically the whole surface of the sample and finishing in a small MVC area. MFM type II (Fig. 2b) is characterized by a FPZ formed by a TTS placed at the surface (as in MFM type I) but extended over a wider arc of sample perimeter. At the beginning fracture is initiated and grows subcritically there and then, at the critical instant, grows by cleavage in divergent directions covering practically the whole surface of the sample and finishing in a not so small MVC area located in the opposite position. MFM type III (Fig. 2c) is identified by a FPZ formed by TTS placed around the whole sample perimeter, i.e., an outer TTS crown does exist. Fracture process initiates and grows subcritically there and afterwards, when a critical state is reached, grows to inner points by convergent cleavage, covering the whole fracture surface. This type was observed preferentially in those steels undergoing a high degree of cold drawing, so it could be a consequence of the marked microstructural anisotropy of these steels. MFM type IV (Fig. 2d) is characterized by a FPZ where a mixture of TTS and MVC* are coexisting. In contrast with the previous cases, this zone is placed in the core regions of the fracture section. Therefore, fracture initiates and grows subcritically there and then, at the critical instant, grows to outer points following radial directions by cleavage, finishing in a small ring with MVC fractography. This type of MFM only appears in blunt notch samples D under slow crosshead speed (0.001 mm/min).