PSI - Issue 26

Jesús Toribio et al. / Procedia Structural Integrity 26 (2020) 360–367 Toribio / Structural Integrity Procedia 00 (2019) 000 – 000

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4.2. Identification and characterization of the pearlitic pseudocolony In order to identify and characterize the new (non-conventional) pearlitic pseudocolony in heavily cold drawn pearlitic steel, this section of the paper describes its geometrical and micromechanical (or nanomechanical) features: 1. The pearlitic pseudocolony itself is extremely slender and fully oriented in the direction of cold drawing (wires axis), as the rest of conventional colonies. 2. The pearlite (ferrite//cementite) lamellae inside the pearlitic pseudocolony are not oriented in the direction of cold drawing (wire axis), i.e., they are quasi-perpendicular to the pseudocolony containing them. 3. As a consequence of the previous fact, the area inside the pearlitic pseudocolony exhibits an anomalous ( very high ) local interlamellar spacing (quite higher than the average interlamellar spacing of the steel). 4. Also as a consequence of the second characteristic, curling of the pearlite (ferrite//cementite) lamellae inside the pearlitic pseudocolony is a frequent phenomenon. 5. Local fracture by shear of the pearlite (ferrite//cementite) lamellae ( micro-fracture; breaking of the lamellae ) inside the pearlitic pseudocolony often appears in addition to the curling effect. 6. These characteristics of the pearlitic pseudocolonies make them preferential fracture units with minimum local resistance, thereby producing anisotropy in fracture resistance ( strength anisotropy ). 7. As a consequence of such a strength anisotropy, there is a subsequent effect in the form of crack path deflection associated with mixed mode propagation . 4.3. Role of pearlitic pseudocolonies promoting anisotropic fracture The role of pearlitic pseudocolonies promoting anisotropic fracture and crack path deflection in heavily cold drawn pearlitic steel has been studied thoroughly by Toribio et al. (1997), Toribio and Ayaso (2001, 2002a, 2002b, 2003, 2004) and Toribio (2004b, 2008). As described by Toribio (2004b), transversely pre-cracked rods were subjected to monotonic tensile loading up to fracture. Fig. 7 shows the propagation profile for a hot rolled bar (not cold drawn at all) and for a heavily cold drawn pearlitic steel (commercial prestressing steel wire whose markedly oriented microstructure is shown in Fig. 4).

II F

f

f I

F

(a)

(b)

Fig. 7. Crack paths (propagation profiles) produced by axial fracture in inert (air) environment in steels with 0 (a) and 6 (b) cold drawing steps; f: fatigue crack growth; I: mode I propagation; II: mixed mode propagation (propagation step in heavily drawn steels); F: final fracture.

The initial hot rolled material and the slightly drawn steels behave isotropically, i.e., cracking develops in mode I following the initial plane of fatigue crack propagation (Fig. 7a). The most heavily drawn steels exhibit a clearly anisotropic fracture behaviour in the form of crack deflection after the fatigue precrack (and some mode I propagation in certain cases) with a deviation angle of almost 90º from the initial crack plane and further propagation in a direction close to the initial one (Fig. 7b). Fig. 8 sketches the micromechanics of fracture in the case of heavily cold drawn pearlitic steel; as explained in previous paragraphs, the pseudocolonies represent weakest units promoting fracture and crack deflection appears when the macroscopic fracture path reaches a pearlitic pseudocolony, that itself fails by shear cracking of pearlite if not previously pre-cracked or pre-damaged as a consequence of the manufacture process by cold drawing, thus producing curling and micro-fractures .