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This paper presents a finite element study of the hydrogen effects on macroscopic deformation to understand mechanistically the contradictory results of experimental observations. Despite extensive research, there have been many controversies on whether hydrogen hardens or softens iron and steels. Moreover, the conventional application of hydrogen-enhanced localized plasticity (HELP) theory - including a decrease in the local flow stress in the presence of hydrogen - results in an expansion in the plastic zone ahead of a blunting crack tip rather than the localization of plastic deformation which has been reported in many experimental observations. Therefore, we propose a physical model to interpret the criterion for the application of local softening concept. According to our physical model, called pinning-softening model, the hydrogen-induced softening merely occurs in the large shear stress regions, e.g. in the vicinity of the crack tip. In such regions enough shear stress to overcome the critical shear stress for dislocation glide exists. The remote areas from the stress raisers do not satisfy the critical condition of slip; as such, hydrogen-induced hardening occurs rather than softening. Numerical results show that our new model successfully satisfies the concept of HELP theory which is based on the localization of slip bands and increasing the amount of plastic deformation that occurs in a localized region adjacent to the fracture zone. The pinning-softening model not only explains the contradictory results of hydrogen effects on the macroscopic deformation, but also gives more insight into the mechanistic understanding of hydrogen embrittlement phenomenon.
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