Enhanced hydrogen concentrations ahead of rounded notches and cracks-competition between plastic strain and hydrostatic stress

The finite element method was used to solve the coupled elastic-plastic boundary value problem and transient hydrogen diffusion initial boundary value problem. Solutions were obtained at room temperature and under plane strain deformation in the neighborhood of a blunting crack tip under small scale yielding conditions and in the neighborhood of a rounded notch in a four-point bend specimen. The hydrogen population profiles in both normal interstitial lattice sites (NILS) and trapping sites were calculated and conditions for the predominance of the total amount of hydrogen by either of the populations were studied. A discussion of the finite element results in conjunction with different mechanisms of hydrogen embrittlement is presented. If a critical amount of hydrogen is required for hydrogen induced crack initiation, the present results predict locations of crack initiation sites at steel bend specimens which are in agreement with experimental observations on the occurrence of the first microcracking event.