INFLUENCE OF FATIGUE PRE-LOADING LEVEL ON HYDROGEN-ASSISTED MICRO-DAMAGE IN EUTECTOID STEEL

Previous research on hydrogen induced 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 micro-damage [1]. The TTS mode appears in fracture tests on precracked and notched specimens when tested under hydrogen charging. Experimental results [2] showed phenomenological relations between the size of the TTS region and variables such as the electrochemical potential and the maximum stress intensity factor during fatigue precracking (for cracked samples), or the time to failure and the geometry (for notched samples). A hydrogen diffusion model was proposed to explain these relations [3].

In this paper a fracture mechanics approach to hydrogen-assisted micro-damage in eutectoid 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 principles. In this case, the change from TTS to cleavage takes place when a critical stress intensity factor (KH) 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 KH depends on the fatigue pre-cracking regime and its value may be associated with a characteristic level of stress intensity factor in the crack growth kinetics curve.

[1] J. Toribio, A.M. Lancha and M. Elices, Characteristics of the New Tearing Topography Surface. Scripta Metall. Mater. 25 (1991) 2239-2244.
[2] J. Toribio, A.M. Lancha and M. Elices, Hydrogen Embrittlement of Pearlitic Steels: Phenomenological Study on Notched and Precracked Specimens. Corrosion 47 (1991) 781-791.
[3] J. Toribio, A.M. Lancha and M. Elices, The Tearing Topography Surface as the Zone Associated with Hydrogen Embrittlement Processes in Pearlitic Steel. Metall. Trans. 23A (1992) 1573-1584.