Hydrogen transport near a blunting crack tip

The hydrogen transport model of Sofronis and McMeeking was used in order to simulate the effect of the hydrostatic stress and trapping on the hydrogen distribution in a plastically deforming steel. In this model it is assumed that hydrogen atoms diffuse through lattice sites and that trap sites are filled by lattice diffusion. These trap sites are formed due to plastic deformations. Coupled diffusion elastic-plastic finite element analyses were carried out in order to investigate the hydrogen concentration in lattice and trap sites near a blunting crack tip under small-scale yielding conditions. The numerical results of Sofronis and McMeeking were reproduced and it was found that in their model hydrogen is created. The hydrogen balance is satisfied by including a strain rate factor in the hydrogen transport equation. As a consequence no differences were found at steady state, i.e. at low strain rates. The strain rate factor decreases the hydrogen concentration in lattice sites due to the filling of trap sites. When the strain rate is sufficiently high, the lattice sites can be almost depleted of hydrogen while trap sites remain saturated. The modified hydrogen transport model predicts strong dependence of the hydrogen concentration in lattice sites on the strain rate, while the hydrogen concentration in trap sites is not affected significantly. The modified hydrogen transport model provides greater insight into the strain rate dependence of hydrogen embrittlement as observed in tensile tests. (C) 1999 Elsevier Science Ltd. All rights reserved.