PSI - Issue 28

Jesús Toribio et al. / Procedia Structural Integrity 28 (2020) 2416–2423 Jesús Toribio et al. / Procedia Structural Integrity 00 (2020) 000–000

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1. Introduction Cold drawn pearlitic steel wires are widely used as components of pretressed concrete structures in civil engineering construction. They can be considered as high-performance and hierarchically-structured materials (Toribio, 2006; Zhang et al., 2013; Borchers and Kirchheim, 2016; Toribio, 2017a; Toribio, 2017b; Toribio, 2018a) whose really high strength is only limited by cleavage fracture, as described by Gil-Sevillano (1986). 2. Cold drawing of pearlitic steels Manufacturing of prestressing steel wires is made by progressive cold drawing to increase the yield strength and the ultimate tensile strength (UTS) by activating a strain hardening mechanism. Fig. 1 shows the stress-strain curves of progressively cold drawn pearlitic steels wires from A0 (hot rolled material, not cold drawn at all) to commercial prestressing steel wire A6 that has undergone six steps of cold drawing.

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1.5

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0.0 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 A6 A5 A4 A3 A2 A1 A0  (GPa)  0.5

Fig. 1. Stress-strain curves of progressively drawn pearlitic steels A0-A6 (from 0 to 6 drawing steps).

3. Microstructural evolution with cold drawing Previous research by Toribio and Ovejero (1997, 1998a, 1998b, 1998c) showed the progressive slenderizing and orientation (along the wire axis or cold drawing direction) of the pearlitic colonies (first microstructural level), as well as increasing orientation (again along the wire axis or cold drawing direction) together with densification of the ferrite/cementite lamellae (second microstructural level) linked with a decrease of pearlite (ferrite/cementite) interlamellar spacing. Thus the microstructure of the cold drawn pearlitic steel wire becomes progressively oriented as the cold-drawing degree increases. 4. Notch-induced anisotropic fracture behavior In the matter of the anisotropic fracture behavior and the associated crack path deflection and mixed mode state taking place in cold drawn pearlitic steel in the presence of notches ( notch-induced anisotropic fracture behavior ), large research work was performed by Toribio and Ayaso (2001, 2002a, 2002b, 2003a, 2003b, 2004a, 2009) on notched samples of progressively cold drawn pearlitic steels with very different notch geometries (Fig. 2) and thus very distinct stress triaxiality (constraint) levels were analyzed with regard to their role in fracture behavior. Notched specimens are very adequate for analyzing fracture processes in structural materials and formulate fracture criteria (Toribio, 1997). In addition, they exhibit many advantages in the study of hydrogen embrittlement processes in materials where hydrogen transport by stress-assisted diffusion plays a relevant role (Toribio and Ayaso, 2004b).