PSI - Issue 41

Jesús Toribio et al. / Procedia Structural Integrity 41 (2022) 712–717 Jesús Toribio / Procedia Structural Integrity 00 (2022) 000–000

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2.2. Micro-crack path profile Toribio (2018) recently provided a summary of previous research on microstructure-based anisotropic fatigue behavior of hot rolled and cold drawn pearlitic steel wires and the corresponding crack path. Fig. 3 shows the fatigue micro-crack path profiles in progressively cold drawn pearlitic steel wires (from left to right, steels E0, E2 and E6, where the digit indicates the number of cold drawing steps undergone by each steel wire). Fatigue micro-crack paths are trans-colonial and trans-lamellar, containing non-uniform crack opening displacements, micro-discontinuities, branchings, bifurcations and local deflections, thereby creating microstructural roughness , as sketched in Fig. 4 that includes the fatigue crack paths ( micro-crack path profiles ) in the hot rolled bar E0 (left) and the cold drawn wire E7 (right), the latter undergoing seven cold drawing steps during manufacturing. Therefore, progressively cold drawn pearlitic steel wires exhibit a locally multi-axial fatigue behaviour .

Fig. 3. Fatigue micro-crack path profiles in progressively cold drawn pearlitic steel wires (from left to right, steels E0, E2 and E6, where the digit indicates the number of cold drawing steps undergone by each steel wire).

d (7)

d (0)

θ (7)

θ (0)

Fig. 4. Sketches of the fatigue crack paths (micro-crack path profiles) in the hot rolled bar E0 (left) and the cold drawn wire E7 (right).

The cold drawn wire exhibits a fatigue crack path with more micro-deflections than the hot rolled bar (Fig. 4), the micro-deflection path being shorter and the micro-deflection angle being higher in the former than in the latter, i.e.,

(0) (7)   

(1) (2)

(0) (7) d d 

Thus the frequency of deflections in the fatigue crack path increases with the cold drawing degree (Fig. 4). This tortuous crack path is one explanation of the improvement of fatigue performance in the cold-drawn wire when compared to that of the hot rolled bar (Toribio and Toledano, 1999, 2000; Toribio et al ., 2007, 2008). The physical crack paths of Fig. 3 show evidence of different events: (i) microstructurally-induced crack paths, (ii) crack paths micro-roughness, (iii) crack-path micro-deflections, (iv) locally multi-axial crack paths, (v) kinked crack paths; (vi) deflected crack paths, (vii) bifurcated crack paths. With regard to micro-crack path profiles in the close vicinity of the crack tip, near-tip crack deflection and bifurcation are relevant issues. Two recent papers by Toribio et al. (2021a, 2021b) address these problems from the numerical point of view, on the basis of finite-element analyses.