PSI - Issue 41

Jesús Toribio et al. / Procedia Structural Integrity 41 (2022) 718–723 Jesús Toribio / Procedia Structural Integrity 00 (2022) 000 – 000

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1. Introduction Material microstructure plays a relevant role in the matter of macroscopic fatigue behavior, in addition to the key importance of the specific crack configuration (Kitagawa et al.,1975; Suresh, 1983). With regard to the case of ferritic-pearlitic microstructures, studies were performed by Korda et al. (2006a, 2006b) and Mutoh et al. (2007). In the matter of fully pearlitic microstructures under fatigue loading, previous scientific research was developed by Toribio and Toledano (1999, 2000) and Toribio et al. (2009, 2014, 2015, 2017). This scientific paper offers an innovative approach to fatigue crack growth in pearlitic steels that exhibit a microstructurally-induced tortuous propagation path ( zig-zag fatigue crack propagation path), by considering a modified Paris Law including the real fatigue crack growth length (i.e., the locally multiaxial, microstructurally tortuous, fatigue crack growth distance) instead of the apparent fatigue crack growth length (i.e., the globally uniaxial, fatigue crack propagation distance, projected in the direction of global fatigue propagation in mode I). 2. Materials and microstructure As described by Toribio (2018) and Toribio et al. (2020), the materials used were two pearlitic steels: (i) a hot rolled bar (non cold drawn at all); (ii) a cold drawn wire (commercial prestressing steel wire) that has undergone seven cold drawing steps up to reaching a cumulative plastic strain ε P =1.57 and a posterior stress-relieving treatment to eliminate, or at least diminish, residual stresses. Steel was supplied in form of wires with circular section, the diameter ranging respectively between 11 and 5 mm for the hot rolled bar and the cold drawn wire. With regard to the influence of the manufacturing process on the mechanical properties of the pearlitic steel under consideration, the cold drawing process activates in the material a strain hardening mechanism, so that it produces a clear improvement of conventional mechanical properties (Table 1) obtained from a standard tension test: both the yield strength ( σ Y ) and the ultimate tensile strength (UTS, σ R ) increase with cold drawing, while the Young’s modulus ( E ) remains constant and the strain at UTS ( ε R ) decreases with it. Table 1. Mechanical properties of the hot rolled bar and the cold drawn wire. Steel E (GPa) σ Y (MPa) σ R (MPa) ε R Hot rolled bar 202 700 1220 0.078 Cold drawn wire 209 1480 1820 0.060 3. Microstructural evolution with cold drawing Whereas the microstructure of the hot rolled pearlitic steel bar is randomly oriented, the fully cold drawn pearlitic steel wire exhibits a markedly oriented microstructure (in the matter of both colonies and lamellae), fully oriented quasi-parallel to the wire axis or cold drawing direction, as shown in Fig. 1.

Fig. 1. Microstructures of the hot rolled bar (left) and the cold drawn wire (right) in longitudinal sections. Vertical side of the micrograph is always parallel to the wire axis or drawing direction, whereas horizontal side is associated with the radial direction of the cylinders.