PSI - Issue 41

Jesús Toribio et al. / Procedia Structural Integrity 41 (2022) 724–727 Jesús Toribio / Procedia Structural Integrity 00 (2022) 000–000

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1. Introduction Hydrogen-assisted micro-damage (HAMD) in pearlite takes the form of tearing topography surface (TTS), a term coined by Thompson and Chesnutt (1979) and Costa and Thompson (1982). This paper revisits previous research by the author on the topic of hydrogen embrittlement (HE) behaviour in high-strength eutectoid steels with pearlitic microstructure. 2. Hydrogen effects on pearlitic microstructures Previous research by Toribio et al. (1991, 1992) on HE of high-strength eutectoid pèarlitic steels showed that degradation at the micro-scale (HAMD) in pearlite-based microstructures takes place in the form of a special micro fracture mode or hydrogen damage topography (HDT). It is a non-conventional fractographic mode: the so-called tearing topography surface (TTS), as identified by Thompson and Chesnutt (1979) and Costa and Thompson (1982), consisting mainly of micro-tearing at the finest microscopic level, as shown in Fig. 1. Toribio and Vasseur (1997) went further in the analysis and analyzed the evolution of HAMD, describing another type of it (another sub-mode of cracking enhanced by hydrogen): a kind of microscopic mode of fracture resembling micro-void coalescence (MVC), a candidate to TTS (or ‘‘potential’’ TTS region) which does not reach such a condition because of an insufficient level of hydrogenation. Toribio (1997) performed a fracture mechanics approach to HAMD in pearlitic steel, demonstrating that the extension (progress) of this micro-damage in cracked specimens can be modeled as a macroscopic crack prolonging the original fatigue precrack and involving linear elastic fracture mechanics (LEFM) principles. In this case, the change from HAMD in the form of TTS to brittle cleavage-like topography takes place when a critical stress intensity factor ( K H ) is reached, and this value depends on the amount of hydrogen which penetrated the vicinity of the actual crack tip (the fatigue precrack plus the TTS area). More recently, Toribio (2012) reviewed previous analyses and denoted the transition zone between the TTS and the conventional cleavage-like (fast brittle fracture without any need of environmental assistance) as quasi-MVC, describing the sequence of HAMD as TTS, quasi-MVC and final fracture by cleavage.

Fig. 1. Hydrogen-assisted micro-damage (HAMD): the hydrogen damage topography (HDT) is tearing topography surface (TTS).

3. Phenomenological aspects The TTS area is not due to hydrogen alone but requires the combined action of stress-strain fields, so that the fracture takes place when a critical combination of hydrogen concentration and stress (or strain) is achieved. However, there is clear experimental evidence of the association between the TTS and the presence of hydrogen, as discussed by Toribio et al. (1991, 1992). A summary of experimental results supporting this assumption is given in the following paragraphs: