A new model for hydrogen-induced cleavage crack nucleation

Abstract

The mechanisms of hydrogen promoting cleavage crack nucleation have been the subject of research for many years. Wang et al. [1] proposed a mechanism for hydrogen promoting microcrack nucleation in bcc metals. His idea was that the nucleation of a cleavage crack in bcc metals is the dislocation reaction proposed by Cottrell [2]. The core of the gliding dislocation should be saturated with hydrogen, and the dislocation will move along with the hydrogen atmosphere around it. When the two sets of gliding dislocations coalesce and form a cleavage microcrack consisting of a set of the sessile dislocation, the hydrogen atoms carried by the gliding dislocations will enter and be enriched into the microcrack, and the hydrogen pressure in the cleavage microcrack can promote the formation of a stable cleavage crack [1]. Chu et al. recently developed the formula for the threshold stress intensity of cleavage crack initiation [3]. The stress for initiation of the cleavage crack can be reduced through the cooperation of hydrogen induced local plastic deformation and hydrogen decrease of the cohesive strength. Because the formation of a cleavage crack by Cottrell's dislocation reaction mechanism described by Wang et al. [1] can only be applied to bcc metals, this mechanism cannot be used to explain cleavage crack formation in fcc or hcc metals. Although the mechanism for hydrogen promoting microcrack formation in the literature [3] can be used in various structures, it is only the stress condition for microcrack initiation. The aim of the present work was to propose a new mechanism of hydrogen induced cleavage crack nucleation by considering energy changes during cleavage crack nucleation. Most experimental observations have shown that even for brittle materials, a cleavage crack can be nucleated only when local plastic deformation develops to the critical condition [4]. Local plastic deformation leads to the formation of dislocation pile-ups. According to Stroh's calculation [5, 6], the maximum stress of the group of piled-up dislocations is: