Hydrogen-microvoid interactions at continuum scale

Abstract

Experimentally, it has been shown that hydrogen can either enhance the internal necking failure or induce internal shearing failure of microvoids. In this study, the numerical investigation of hydrogen-microvoid interactions under the framework of hydrogen enhanced localized plasticity mechanism reveals that the actual effect of hydrogen depends on the stress state as well as on the hydrogen trapping effect. Hydrogen enhanced internal necking failure is observed over the entire stress space at a low level of trapping effect. While such failure is still observed in the high triaxiality regime at a high level of trapping effect, hydrogen induced internal shearing failure is observed in the low triaxiality regime. A hydrogen induced internal shearing failure criterion is proposed, and the failure loci corresponding to the low and high levels of trapping effect are constructed.The hydrogen induced internal shearing failure locus is found to be approximately independent of stress triaxiality while the hydrogen enhanced internal necking failure locus maintains similar triaxiality dependency as in the absence of hydrogen. The loss of ductility, in terms of reduction in failure strain and dimple size, is more pronounced in the case of hydrogen induced internal shearing failure. Subsequent study of the Lode effect reveals that plane strain tension is the most critical case for hydrogen induced internal shearing failure while axisymmetric tension is the most critical for hydrogen enhanced internal necking failure.

Extraordinary Response of H-Charged and H-Free Coherent Grain Boundaries in Nickel to Multiaxial Loading

“…Since the plane of Σ3 GB is also the plane of dislocation glide and many of fractured Σ3 GBs were inclined by 60 • to the tensile axis, the authors assumed that a shear deformation by dislocation glide in the GB plane assisted the crack initiation process, along with dislocations trapped to this plane from other (111) slip planes. Such a damage process resembles the shearing failure induced by the HELP mechanism at low triaxialities [16]. Further crack propagation followed along general grain boundaries due to their highest energy (lowest separation energy and cohesive strength) and the highest hydrogen concentration.…”

“…The HEDE concept deals with a hydrogen-induced reduction of the cohesive strength of grain boundaries (GBs), while the HELP model is based on the hydrogen-enhanced dislocation mobility. At high tensile triaxialities, the HELP mechanism accelerates the necking failure of microvoids and, at low triaxialities, it induces their shearing coalescence and failure (see e.g., [16,17]). The relevance of HEDE and HELP damage mechanisms should be identified for each particular case of the hydrogen-assisted fracture.…”

Numerical study of hydrogen influence on void growth at low triaxialities considering transient effects

“…For example, = 0.5, initial yield stress might be reduced only to the 50 % in the presence of hydrogen. Yu et al [15] argued that a linear softening could not lead to a shearing failure and they replaced it with a sigmoidal softening law.…”

“…Ahn et al [14] have established a consistent strategy to implement an informed cohesive model in which Traction Separation Laws (TSL) are based on the behaviour of a void unit cell within a hydrogen environment producing local softening. Within that framework, Yu et al [15] modified the softening law in order to consider a more realistic shearing failure induced by hydrogen and obtained a failure loci for different triaxialities.…”

Simulating hydrogen in fcc materials with discrete dislocation plasticity

“…Barrera et al [19] applied the idea to study hydrogen effects on a notched tensile plate using a coupled mechanical-diffusion analysis. Recently, Yu et al [20] performed a unit cell analysis and obtained a quantitative description of hydrogen failure loci. In these studies, the hydrogen softening effect is still implemented in a phenomenological manner.…”