PSI - Issue 59

Jesús Toribio et al. / Procedia Structural Integrity 59 (2024) 90–97 Jesús Toribio / Procedia Structural Integrit y 00 (2024) 000 – 000

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1. Introduction Previous research on hydrogen-assisted fracture of pre-cracked and notched samples of high-strength pearlitic steel demonstrated the existence of a non conventional microscopic fracture mode: tearing topography surface (TTS), which can be associated with hydrogen-assisted microdamage (Toribio et al., 1991a). The TTS mode appears in fracture tests on pre-cracked and notched specimens when tested under hydrogen charging. Experimental results (Toribio et al., 1991b) showed phenomenological relations between the size of the TTS region and variables such as the electrochemical potential and the maximum stress intensity factor (SIF) during fatigue pre-cracking (for cracked samples), or the time to failure and the geometry (for notched samples). A hydrogen diffusion model was proposed to explain these relations (Toribio et al., 1992). In this paper a fracture mechanics approach to hydrogen-assisted micro-damage (HAMD) in eutectoid pearlitic steel is presented. The TTS area can be modelled as a macroscopic crack that extends the original fatigue pre-crack and involves linear elastic fracture mechanics (LEFM) principles. In this case, the change from TTS to cleavage takes place when a critical SIF in hydrogen environment ( K H ) is reached, and this value depends on the amount of hydrogen which penetrated the vicinity of the actual crack tip (the fatigue pre-crack plus the TTS area). It is shown that the value K H depends on the fatigue pre-cracking regime and its value may be associated with a characteristic stress intensity level in the crack growth kinetics (CGK) curve d a /d t - K . 2. Hydrogen embrittlement (HE) tests The analysis is based on experimental results of hydrogen embrittlement (HE) tests on pre-cracked cylindrical samples of eutectoid pearlitic steel whose chemical composition and mechanical properties appear in Tables 1 and 2. Slow strain rate tests (SSRT) – at very low strain rate – were conducted under simultaneous hydrogen charging by cathodic polarization in aqueous solution, as described elsewhere (Toribio and Lancha, 1993; 1996).

Table 1. Chemical composition (wt %) of the steel. C Mn Si P S

Cr

Ni

Mo

0.74

0.70

0.20

0.016

0.023

0.01

0.01

0.001

Table 2. Mechanical properties of the steel. Young's Modulus E (GPa) Yield Strength  Y (MPa) UTS  R (MPa)

Fracture Toughness K IC (MPam 1/2 )

Ramberg-Osgood parameters*  p < 1.07  p > 1.07 P I (MPa) n I P II (MPa)

n II 17

195

725

1300

53

2120

5.8

2160

* P,n: Ramberg-Osgood Parameters  =  e +  p =  / E +(  / P ) n

The main results are reproduced in a previous paper (Toribio et al., 1992) where phenomenological relations are established between the fracture load in hydrogen F H (divided by its reference value in air F O ) and testing variables of an electrochemical nature (pH and potential E) and mechanical character ( K max / K O ): F H /F O = f (pH, E, K max /K O ) (1) where K max is the maximum SIF at the end of the last stage of fatigue pre-cracking (just before the SSRT) and K O the fracture toughness of the material, obtained using the same type of cylindrical pre-cracked samples as those used in the HE tests . With regard to the TTS zone, Fig. 1 shows the scheme of the three regions detected in a fracture surface of pre cracked specimens tested under simultaneous hydrogen charging: the fatigue pre-crack, a transition topography