Towards the prediction of hydrogen–induced crack growth in high–graded strength steels

This work addresses a hydrogen–induced cracking methodology incorporating the fully coupled problems of elastoplastic deformation and hydrogen transport within cohesive–interface elements framework to predict ductile tearing in high strength steels. In the present work, on one hand, the flow stress declines with escalating hydrogen concentration and, as a result, taking into account the hydrogen impact on enhancing plastic deformation in the lattice through the hydrogen enhanced localized plasticity (HELP) model. On the other hand, to implement hydrogen impact into the cohesive–interface elements in the hydrogen enhanced decohesion (HEDE) manner, a phenomenological decohesion model is adopted, leading to combining the both mechanisms (HELP + HEDE). Furthermore, a nonlinear traction–separation relationship based on the Park–Paulino–Roesler (PPR) model determines the constitutive response of the zero thickness cohesive–interface elements while the finite–strain, incremental plasticity model is accounted for the bulk material. The hydrogen–degraded PPR model provides a flexibility and control over various softening behaviours, from a convex brittle to a concave ductile shape. This computational framework is established to simulate ductile crack extension in a C(T) specimen made of AISI 4130 high strength steels. Afterwards, the parameters involved in the hydrogen–degraded PPR model are properly calibrated with experimental data for the uncharged and hydrogen–charged C(T) specimens. The key contributions of this study are to shed light on the hydrogen concentration (lattice and trapped) effects on the crack growth resistance curves and stress triaxiality. Based on the results obtained for AISI 4130 high strength steels, it has been concluded that the lattice hydrogen has the dominating factor in the hydrogen degradation compared with trapped hydrogen.