Fracture mechanics approach to hydrogen-assisted microdamage in eutectoid steel
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INTRODUCTION
PREVIOUS
research on hydrogen-induced fracture of high-strength pearlitic eutectoid steel[1] revealed the presence in the process zone of a nonconventional microscopic fracture mode associated with hydrogen-assisted microdamage. This particular micromechanical mode was identified as tearing topography surface (TTS),[2,3] and it appears in fracture tests on precracked and notched specimens under simultaneous hydrogen charging, preceding the unstable fracture mode of cleavagelike type. This article offers a mechanical modeling of the TTS microfracture mode in precracked specimens, by considering the progressive extension of the TTS zone as a macroscopic crack that extends the original fatigue precrack involving fracture mechanics principles. In this case, the change from TTS to cleavagelike topography takes place when a critical stress intensity factor (SIF) is reached. An inquiry is made into whether the critical SIF in hydrogen is close to the fracture toughness of the material in air or whether it depends on the amount of hydrogen which penetrates the vicinity of the actual crack tip (the actual crack is the fatigue precrack plus the TTS region). If the critical SIF is a direct function of the hydrogen concentration distribution, it seems plausible that the point of transition from TTS to cleavage in quasi-static tests should be that of maximum hydrostatic stress, since local hydrogen concentration is maximum at that point, and the final fracture process is not purely mechanical but environmentally assisted. On the other hand, if the critical SIF is close to the fracture toughness of the material in air, final fracture by cleavage is of a purely mechanical nature. Clarifying this will provide insight into the physical aspects of stage III (or transition between stages II and III) of the crack growth kinetics curve and the real intrinsic character of such a curve as a design tool in structural integrity engineering.
J. TORIBIO, Full Professor of Materials Science and Engineering, and Head, is with the Department of Materials Science, University of La Corun˜a, 15192 La Corun˜a, Spain. Manuscript submitted April 3, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
II.
PHENOMENOLOGICAL ASPECTS
The analysis is based on experimental results[4] of hydrogen embrittlement tests on precracked cylindrical samples of eutectoid pearlitic steel whose chemical composition and mechanical properties appear, respectively, in Tables I and II. Slow strain rate testing—at very low strain rate—was conducted under simultaneous hydrogen charging by cathodic polarization in aqueous solution, as described elsewhere.[4] The main results are reproduced in Figure 1, and they show phenomenological relations between the fracture load in hydrogen FH (divided by its reference value in air FO) and testing variables of an electrochemical nature (pH and potential E ) and mechanical character (Kmax/KO): FH /FO 5 f (pH, E, Kmax/KO)
[1]
where Kmax is the maximum SIF at the end of the last stage of fatigue precracking and KO the frac
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