PSI - Issue 42

Hiroshi Nishiguchi et al. / Procedia Structural Integrity 42 (2022) 1442–1448 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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3.3. Mechanisms of crack growth and residual stress reduction While significant research effort has focused on the elucidation of the hydrogen embrittlement mechanism, no firm theory of hydrogen embrittlement has been established to date. However, the following theories have been regarded as plausible explanations for hydrogen embrittlement: (1) the hydrogen-enhanced strain-induced vacancy (HESIV) theory emphasizes the importance of a high density of atomic vacancies introduced by hydrogen (Nagumo 2001). (2) The hydrogen-enhanced localized plasticity (HELP) theory states that hydrogen weakens the interactions between the dislocations by the reduction of the dislocation elastic fields, thereby promoting localized plastic deformation. (3) The hydrogen-enhanced decohesion (HEDE) theory focuses on the reduction of atomic bonding forces by hydrogen. Troiano (1960) and Oriani et al. (1974) were among the many researchers that presented experimental results supporting the HEDE theory. Oriani et al. conducted delayed fracture tests using wedge-opening loaded specimens of 4340 steel with a yield stress of 1700 MPa in hydrogen gas. The stress intensity factor at which crack propagation ceases in hydrogen gas, i.e., the lowest stress intensity factor for hydrogen-assisted cracking ( K IH ), was obtained from constant displacement tests under various hydrogen gas pressures. They estimated not only the amount of hydrogen aggregating around the crack tip but also the decrease in the interatomic bond strength due to hydrogen. For the HELP theory, Beachem et al. (1972) proposed for the first time that hydrogen can promote plastic deformation, and Ferreira et al. (1998) used in-situ TEM observations to experimentally demonstrate that hydrogen increases dislocation mobility. The HESIV theory states that hydrogen facilitates the formation and cohesion of atomic vacancies during plastic deformation, and the micro-voids generated at the point of high vacancy density connect with the main crack and contribute to fracture. In addition to the above theories, Matsuoka et al. proposed a theory of hydrogen-enhanced successive fatigue crack growth (Matsuoka et al. 2011, 2016). In this theory, the acceleration of embrittlement by hydrogen is not due to either the presence or absence of hydrogen at the crack tip, but rater is due to the distribution of hydrogen near the tip of the fatigue crack. In other words, a steep hydrogen concentration gradient at the crack tip causes a localization of plasticity that prevents crack-tip blunting and sharpens the crack tip. As a result, the crack growth per cycle is increased. As previously shown, based on the dislocation shielding theory, the crack tip tends to close due to the internal compressive stress arising from the dislocations around a crack tip. Considering the experimental results in the present study, this compressive residual stress field around a crack tip, such as in fatigue cracks, may be reduced by the introduction of hydrogen, and the crack propagation should be promoted in a hydrogen gas environment due to the reduction of the crack closure effect. In the plastically deformed area around the indentation, a large internal stress field, as well as dislocations, are generated, and a mechanical equilibrium state should be established. As was observed from Fig. 7, the residual compressive stress around the indentation was decreased upon the introduction of hydrogen, suggesting the occurrence of breakdown in the mechanical equilibrium state around the indent by the introduced hydrogen. It is well-known that hydrogen atoms interact strongly with elastic fields, indicating that hydrogen has a strong effect on the internal stress fields of dislocations. In fact, Birnbaum et al. (1994) reported that the interactions of dislocations must be weakened by hydrogen, thus enhancing their motion (Robertson 2001). Plastic deformation around a crack tip increases fracture toughness, which is attributed to the crack-tip shielding effect by the dislocations, i.e ., the effect of the internal stress field of the dislocations. Therefore, a decrease in the internal stress fields of the dislocations by hydrogen should cause a significant decrease in the crack-tip shielding effect, resulting in lower fracture toughness. It is still possible that the decrease in the residual stress observed in this study may be caused by the stress relief because of a small amount of crack propagation during hydrogen exposure. However, a crack propagation of approximately 8% (see Fig. 4) would unlikely cause compressive residual stress to decrease by approximately 300 MPa. Future investigations of the relationship between the effect of hydrogen on the residual stress and crack propagation are needed to fully resolve this question. 4. Conclusion This study investigated the effects of high-pressure hydrogen gas exposure on residual stress fields and cracks around the Vickers indentations introduced into {100} silicon wafers. Loads of indentations were 0.98 N and 1.96 N, and the holding time was 30 s. Upon indentation, not only was a large internal stress field formed around the indent,