PSI - Issue 81

Jesús Toribio et al. / Procedia Structural Integrity 81 (2026) 135–139

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2. Materials and effect of cold drawing Progressive cold drawing of pearlitic steel affects the microstructural arrangement in the form of slenderizing of the colonies, decrease of interlamellar spacing and orientation in the direction of cold drawing (wire axis) of both colonies and lamellae (Toribio and Ovejero, 1997, 1998a, 1998b, 1998c), i.e., inducing microstructural anisotropy. 3. Experimental results & discussion With regard to the fracture behaviour of the progressively cold drawn steel wires in the presence of cracks , pre-cracked rods were subjected to tensile loading up to fracture. Fig. 1 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).

II F

f

f I

F

(a)

(b)

Fig. 1. Crack paths (propagation profiles) produced by axial fracture 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. 1a). The most heavily drawn steels exhibit a clearly anisotropic fracture behaviour in the form of crack deflection after the fatigue crack (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. 1b). The fractographic analysis of the deflected crack path (region II in Fig. 1b) linked with anisotropic fracture behaviour shows a sort of elongated and oriented cleavage (see Fig. 2), the elongation and orientation being in the direction of the 90º-propagation step, i.e., quasi-parallel to the wire axis or cold drawing direction.

Fig. 2. Scanning electron micrograph of the 90º-propagation step in steel 6 (propagation from left to right).

Fig. 3 shows the macro-fracture surfaces of the progressively drawn steels. A general evolution may be observed from a typically brittle behaviour with a flat fracture surface in steel 0 (and slightly drawn steels) to a more ductile behaviour with a more or less irregular and stepped fracture surface in steel 6 (and heavily drawn steels). Accordingly, the fracture behaviour evolves from isotropic to anisotropic as the degree of cold drawing increases. The stepped appearance of the fracture surfaces in heavily drawn steels is consistent with their strength anisotropy because there are a lot of step embryos (or meso-steps) and finally one of them becomes the final 90º-step that produces the macroscopic crack deflection. Thus the meso-roughness increases with the degree of cold drawing.