PSI - Issue 28

Jesús Toribio et al. / Procedia Structural Integrity 28 (2020) 2396–2403 Jesús Toribio et al. / Procedia Structural Integrity 00 (2020) 000–000

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In the drawn steel the ductile micro-tears are smaller and with curvier geometry than in the hot rolled bar, due to the microstructural changes (mostly in the cross sectional area of the wire) produced by the high plastic strain undergone by the heavily drawn, and a sort of materials science relationship appears between microstructural unit size and fatigue micro fracture event. Fig. 6 shows fractographic evidence that an increase of the SIF range Δ K also promotes this phenomenon of ductile tearing created by fatigue in both steels. The afore-said TTS microfracture mode has been associated with hydrogen embrittlement in pearlitic steel in previous research works by Toribio (1997), Toribio and Vasseur (1997), Toribio et al. (2004) and Toribio (2012) showing experimental evidence that the TTS area is the embrittled zone in randomly-oriented (or non oriented ) pearlite or the region associated with hydrogen-assisted micro-damage . In addition, following the fracture mechanics approach to the phenomenon of environmentally assisted cracking or stress corrosion cracking (hydrogen assisted cracking in particular) it can be considered a slow propagation mode in the crack growth kinetics curve of hydrogen-assisted fracture processes linked with subcritical cracking at one micrometer per second or less, as analysed by Toribio (1997). In oriented pearlite as a consequence of heavy cold drawing, the hydrogen-assisted micro-damage mode evolves towards a kind of enlarged and oriented TTS (EOTTS) aligned quasi-parallel to the wire axis or cold drawing direction, showing microscopic evidence of both ductile tearing and shear deformation, as described thoroughly in recent papers published by Toribio (2017a, 2017b, 2018, 2020). The micro-tearing events with TTS appearance shown in Figs. 5 and 6 are consistent with plastic crack advance during fatigue (Toribio et al., 2011), a mechanism that is not based on micro-fracture events but on transferring material form the vicinity of the crack tip to the crack flanks, without any necessity of plasticity-induced fatigue crack closure, a really controversial and not widely accepted phenomenon, as discussed elsewhere (Vasudevan et al., 1994; Vasudevan et al., 2001; Sadananda and Vasudevan, 2004; Stoychev and Kujawski, 2005; Noroozi et al., 2008; Toribio and Kharin, 2011; Toribio and Kharin, 2013). Fig. 7 shows the fatigue crack paths in each particular material with non-oriented or randomly oriented (Fig. 7a) and oriented (Fig. 7b) pearlite after heavy cold drawing during manufacturing of prestressing steel wires.

(a) (b) Fig. 7. Fracto-materialographic analysis for an intermediate level of the stress intensity factor range Δ K : (a) hot rolled bar; (b) cold drawn wire, showing the profile of the fatigue crack path in each particular material with non-oriented or randomly-oriented (a) and oriented (b) pearlite. Fatigue fracture in both steels essentially develops by breaking the lamellae inside the colonies, i.e., it can be classified as translamellar and transcollonial (breaking the ferrite/cementite lamellae and crossing the colonies), showing very localized plastic damage., which is consistent with previous numerical approaches to the fatigue phenomenon in high-strength steels (Toribio et al., 2011; Toribio and Kharin, 2011; Toribio and Kharin, 2013). Fatigue cracking takes place in a very tortuous manner, with frequent deflections (many micro-deviations from the main direction of macro-crack advance in mode I) and certain evidence of branches and bifurcations. The afore-said collection of events determines the existence of a local propagation regime with a very marked mode mixity that promotes locally multiaxial fatigue crack propagation . The described phenomena, consisting of recurrent deviations from the main crack propagation path, provokes an increase of surface micro-roughness and, therefore, a decrease of the driving force for fatigue, thus slowing down the fatigue crack advance.