PSI - Issue 59

Jesús Toribio et al. / Procedia Structural Integrity 59 (2024) 137–144 Jesús Toribio / Procedia Structural Integrity 00 ( 2024) 000 – 000

139

3

(Toribio et al., 1991a; 1991b; 1992; 1993; Toribio and Vasseur, 1997; Toribio, 1997; 2012), so that the size of the TTS region is an indicator of the extension of HAMD. Fig. 2 shows the appearance of the TTS zone.

Fig. 1. SSRT results in terms of respective fracture loads in aggressive (hydrogen) and inert (laboratory air) environments as a function of the maximum stress intensity factor (SIF) during fatigue pre-cracking K max (divided by the fracture toughness of the material K IC ). In Fig. 2 a plot is given of the TTS depth vs. K max , showing that the fatigue pre-cracking regime also influences clearly the micromechanics of HAC in the steel. The higher the fatigue pre-cracking load, the lower the extension of the TTS domain and, accordingly, the lower the deleterious effect of hydrogen on metal, which is consistent with trend plotted in Fig. 1, i.e., the increase of failure load in the hydrogen environment for higher K max -values. This phenomenon is a sort of overload retardation effect in HAC.

Fig. 2. Tearing topography surface (TTS) and depth of the TTS zone as a function of the maximum SIF K max during fatigue pre-cracking.

These phenomena are caused by the development of the cyclic plastic zone and the presence of compressive stresses (cyclic residual stresses) in the vicinity of the crack tip as a consequence of the fatigue pre-cracking method. The crack tip is pre-strained (and pre-stressed) by fatigue: the higher the cyclic load level, the more pronounced is the pre-straining/stressing effect, delaying the hydrogen entry into the metal and improving material performance. To ascertain the mechanical effects of the pre-cracking regime on EAC, it is desired to know the evolution of certain mechanical variables associated with the EAC processes. The item of primary interest is the stress distribution beyond the crack tip affected by cyclic pre-loading. In particular, hydrostatic stress plays a fundamental role in HAC processes driven by stress-assisted hydrogen diffusion.