Mesoeffect of the Dual Mechanism of Hydrogen-Induced Cracking

One of the main methods of protecting pipelines and machine parts against stress corrosion and hydrogen embrittlement is to test metals for hydrogen-induced cracking. The hydrogen-induced cracking test is a standard procedure for testing steels and titanium alloys and for studying hydrogen resistance. The experimentally revealed phenomenon of nonuniform hydrogen distribution after hydrogen charging of specimens is called the skin effect. Here we study the influence of the skin effect after hydrogen charging on the crack growth under mechanical stresses and the influence of a 50-mu m-thick skin layer on the durability of bulk specimens. A corset-type cylindrical specimen with a circumferential notch is considered. The Oriani decohesion model is chosen as a hydrogen embrittlement model. The investigation is performed using our data on the real nonuniform hydrogen distribution and the literature data on hydrogen diffusion coefficients, diffusion activation energy, steel parameters, cohesive law parameters, as well as other parameters of the hydrogen embrittlement model proposed by Serebrinsky. Hydrogen redistribution is described by the diffusion law taking into account mechanical stresses. Modeling is carried out with the original finite volume code in the axisymmetric setting. Crack propagation parameters are determined. The fracture pattern is complex. Cracking first occurs by the hydrogen-enhanced decohesion mechanism and then by the conventional mechanism, which explains the experimentally observed brittle-ductile fracture behavior in tensile hydrogen-charged specimens.