PSI - Issue 28

Jesús Toribio et al. / Procedia Structural Integrity 28 (2020) 2424–2431 Jesús Toribio / Procedia Structural Integrity 00 (2020) 000–000

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3. Consequences of microstructural evolution on fracture behaviour In order to discuss the consequences of the aforesaid multiscale microstructural evolution, previous research by Toribio and Ayaso (2002, 2004) will be considered. 3.1. Fracture of cracked specimens As described by Toribio and Ayaso (2002), transversely pre-cracked rods were subjected to monotonic 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 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. 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). With regard to the fractographic analysis of the deflected crack path (region II in Fig. 1b) linked with anisotropic fracture behaviour, a sort of enlarged and oriented cleavage was discovered, the orientation and enlargement being in the direction of the 90º-propagation step, i.e., quasi-parallel to the wire axis or cold drawing direction. 3.2. Fracture of notched specimens Toribio and Ayaso (2004) also studied the anisotropic fracture behaviour of progressively cold drawn pearlitic steels in the presence of notches with very different notch geometries (Fig. 2) and thus very distinct stress triaxiality (constraint) levels. 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. The heavily drawn steel exhibits anisotropic fracture behaviour and a deflected fracture path, cf. Fig. 3a (with a deflection angle of 90º), i.e., a propagation step. The fractographic appearance of this step resembles cleavage-like fracture (Fig. 3b). However, it is not conventional cleavage, but again a sort of enlarged and oriented cleavage , its orientation being parallel to the wire axis, and with river patterns which are detectable in the same direction