Hydrogen-enhanced localized plasticity

The second is the absorbed hydrogen-enhanced local plasticity mechanism (HELP). This is based on the fact that the local decrease of the flow stress by hydrogen leads to highly localized failure by ductile processes, while the local macroscopic deformation remains small. Shear localization results from local hydrogen absorption, giving a macroscopically brittle fracture related to microscopic localized deformation.95... 

Lu G, Zhang Q, Kioussis N, Kaxiras E. Hydrogen-enhanced local plasticity in aluminum An ab initio study. Phys. Rev. Lett. 2001 87 095501. 

Bimbaum HK, Sofronis P. Hydrogen-enhanced localized plasticity—a mechanism for hydrogen-related fracture. Mater. Sci. Eng. A 1994 176 191-202. 

The Absorbed Hydrogen Enhanced Local Plasticity Mechanism (HELP)... 

The hydrogen distribution in a metal under stress is highly non-uniform which can lead to locally increased hydrogen-enhanced plasticity causing local microscopic deformation and eventually a failure. 

A side effect of the anodic dissolution is often, particularly at crack tips (see Sec. 5.2.5), the production of hydrogen by the reduction of water. Some studies have shown that hydrogen absorption can favor local plasticity, due to enhanced dislocation velocities with hydrogen. In the presence of a crack, accelerated hydrogen penetration can occur very near to the crack tip region by stress-assisted diffusion and dislocation transport. These effects will be discussed later. 

Another approach is related to the fact that hydrogen has been shown to locally enhance plasticity (Sec. 4.3). Two models have been proposed in this field, taking into account (i) the adsorption and (ii) the absorption of hydrogen. 

The first suggestion for such a mechanism was given by Beachem (1972) who showed that hydrogen embrittlement of steels is in fact associated with locally enhanced plasticity at the crack tip. The hydrogen-plasticity interactions have already been noted (Sec. 5.4), after the first observations of Bastien and Azou (1951). 

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