Modelling of hydrogen-induced fracture in iron

Abstract

An atomistic model is proposed for hydrogen-induced fracture in metals with very low solid solubility of hydrogen and which show no formation of hydrides on the basis of experimental results obtained on single crystals of pure iron. Hydrogen atoms released from various traps upon loading are redistributed in the stress concentration region at the tip of a sharp crack to form a plate-like cluster with monoatomic thickness, the most energetically favourable form of clustering of interstitial atoms. Analyses are presented a the steadily propagating half-crack with a cluster on the basis of the Peierls approxiamation, which take into account the atomistic force displacement relations between the two neighbouring atomic planes adjacent to the centre of the crack. The calculations predict that the stress intensity factor and the velocity of propagation of the crack decrease as the width of the plate increases. The time delay in the start of crack propagation after loading is explained via the clustering of hydrogen atoms by diffusion. The cluster disappears by outward diffusion of hydrogen atoms through the crack tip upon unloading.