Mechanics models for hydrogen embrittlement mechanisms

Of the many suggestions of hydrogen-induced degradation of engineering materials, two mechanisms appear to be viable in non-hydride forming systems: hydrogen-enhanced localized plasticity (HELP) and hydrogeninduced decohesion. The present work attempts to link the experimentally observed hydrogen-enhanced dislocation mobility and lattice dilatation to shear localization and ductile rupture processes at the macroscale in the presence of hydrogen. Plasticity models accounting for the hydrogen effects at the microscale are used to study the conditions under which hydrogen induces shear banding in a specimen under plane-strain tension. It is demonstrated that hydrogen induces loss of ellipticity in the governing rate equations of the macroscopic material deformation. Studies on the crack-front constraint variations indicate that the resistance to ductile fracture of low and high constraint configurations depends on the initial hydrogen concentration and the associated amount of softening. On the basis of the Rice and Tracey model for void growth, it is demonstrated that the fracture process ahead of a crack tip in the presence of hydrogen is strongly controlled by the plastic strain in agreement with the HELP mechanism for embrittlement. In contrast, one of the earliest and most often cited theories of hydrogen embrittlement is the decohesion theory, which is based on the postulate that solute hydrogen decreases the force required to separate the crystal along a crystallographic plane, grain boundary or a particle/matrix interface. Decohesion along grain boundary carbides is believed to be a form of hydrogen-induced degradation, also observed experimentally in Ni-base alloy 690. A coupled model of transient hydrogen transport through a plastically deforming matrix with elastic precipitates and debonding particle/matrix interfaces is presented. The numerical results indicate that hydrogen reduces both the macroscopic stress and strain for internal void nucleation.