PSI - Issue 28

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Jesús Toribio et al. / Procedia Structural Integrity 28 (2020) 2416–2423 Jesús Toribio et al. / Procedia Structural Integrity 00 ( 020) 0 0–000

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Fig. 2. Notched samples of cold drawn pearlitic steels with very different geometries. The dimensions of the notched geometries used in the analysis (Fig. 2) were as follows: Geometry A : R/D = 0.03, C/D = 0.10 Geometry B : R/D = 0.05, C/D = 0.30 Geometry C : R/D = 0.40, C/D = 0.10 Geometry D : R/D = 0.40, C/D = 0.30 where R and C are respectively the notch radius and the notch depth, and D the diameter of the sample. 5. Fractographic analysis This section offers a macro-and micro-approach to the fractographic analysis, covering both the fracture path and the microscopic fracture modes to correlate, in the sense of materials science and engineering, the microstructure of the material and its macroscopic fracture behaviour. Figs. 3-7 show the fracture profiles in notched samples A, B and C, together with the microstructure of the steels (Toribio and Ovejero, 1997; 1998a; 1998b; 1998c) at the two microstructural levels of the pearlitic colonies ( 1 st microstructural level ) and Fe//Fe 3 C lamellae ( 2 nd microstructural level ). In addition, the fracture propagation step is also represented (Figs. 3d-7d), a fractographic mode described by Toribio and Ayaso (2002c, 2004c), consisting of a sort of oriented and enlarged cleavage mode, the orientation and enlargement being along the drawing direction. A clearly anisotropic fracture behaviour is observed in the most heavily drawn pearlitic steels, associated with the experimental evidence of the appearance of fracture propagation step deflected 90º in relation to the initial fracture propagation direction in mode I (transverse to the wire), Specimen geometry D (with minimum level of stress triaxiality or constraint) does not exhibit any global 90ª-step, but only local deflections ( embryos of anisotropic fracture). Thus the 90º- step (cf. Figs. 3d-7d) is the signal of strength anisotropy induced in heavily drawn pearlitic steels as a result of the orientation (microstructural) anisotropy generated in the material by the cold-drawing manufacture process. 6. Constraint effects on notch-induced anisotropic fracture behavior and crack path deflection in cold-drawn pearlitic steels: on the role of stress triaxiality (constraint) and microstructural anisotropy/orientation The afore-said anisotropic fracture behaviour of heavily cold drawn pearlitic steels (exhibiting a fracture profile with a 90º-step) can be rationalized on the basis of the markedly oriented microstructure of the steel. However, a sufficient level of stress triaxiality ( constraint ) is required in the notched specimens to obtain the afore-said anisotropic fracture behaviour (associated with cracking/fracture path deflection ) and mixed mode propagation and to avoid the otherwise isotropic fracture behaviour for lower levels of stress triaxiality. The limit case of minimum triaxiality (smooth wire) exhibits isotropic fracture behaviour (Toribio et al., 2016), while that of maximum triaxiality (cracked wire) has a clear anisotropic fracture behaviour (Toribio and Toledano, 2000; Toribio, 2002; Toribio, 2004; Toribio et al., 2007a; Toribio et al., 2011; Toribio et al., 2012: Toribio et al., 2013; Toribio et al., 2015) and a directional toughness was defined by Toribio and Valiente (2004, 2006).