Fractographic and numerical study of hydrogen–plasticity interactions near a crack tip
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This paper offers a fractographic and numerical study of hydrogen–plasticity interactions in the vicinity of a crack tip in a high-strength pearlitic steel subjected to previous cyclic (fatigue) precracking and posterior hydrogen-assisted cracking (HAC) under rising (monotonic) loading conditions. Experiments demonstrate that heavier cyclic preloading improves the HAC behaviour of the steel. Fractographic analysis shows that the microdamage produced by hydrogen is detectable through a specific microscopic topography: tearing topography surface or TTS. A high resolution numerical modelling is performed to reveal the elastoplastic stress–strain field in the vicinity of the crack tip subjected to cyclic preloading and subsequent monotonic loading up to the fracture instant in the HAC tests, and the calculated plastic zone extent is compared with the hydrogen-assisted microdamage region (TTS). Results demonstrate that the TTS depth has no relation with the active plastic zone dimension, i.e., with the size of the only region in which there is dislocation movement, so hydrogen transport cannot be attributed to dislocation dragging, but rather to random-walk lattice diffusion. It is, however, stress-assisted diffusion in which the hydrostatic stress field plays a relevant role. The beneficial effect of crack-tip plastic straining on HAC behaviour might be produced by the delay of hydrogen entry caused by residual compressive stresses and by the enhanced trapping of hydrogen as a consequence of the increase of dislocation density after cyclic plastic straining.
KeywordsStress Intensity Factor Plastic Zone Plastic Zone Size Slow Strain Rate Test Hydrogen Transport
The authors wish to thank the financial support of their research at the University of Salamanca provided by the following institutions: Spanish Ministry for Scientific and Technological Research MCYT-FEDER (Grant MAT2002-01831), Junta de Castilla y León (JCYL; Grant SA078/04) and Spanish Foundation “Memoria de D. Samuel Solórzano Barruso”. In addition, the authors wish to express their gratitude to EMESA TREFILERIA S.A. (La Coruña, Spain) for providing the steel used in the experimental programme.