Multiscale modeling of hydrogen transport in steels and its resulting embrittlement effect

Abstract

A multiscale modeling approach is adopted in this study to understand the hydrogen embrittlement (HE) mechanisms and to predict failure of engineering components under the influence of hydrogen environment. Molecular dynamics simulations of the bodycentered cubic (BCC) iron are conducted to examine the theories of hydrogen enhanced localized plasticity (HELP) and hydrogen enhanced decohesion (HEDE). It is shown that hydrogen aggregation at the crack tip and along grain boundary (GB) reduces the surface energy for creating new crack surfaces, leading to changes in fracture modes caused by preemptive crack propagation. At the continuum level, a numerical framework is developed, which incorporates hydrogen transport in steels and the resulting HELP and HEDE mechanisms into a finite element phase field model to predict crack initiation and propagation in engineering components. As an example, a compact tension (CT) specimen made of a pipeline steel is analyzed. The numerical model captures the phenomenon of hydrogen aggregation occurring proximal to the crack tip driven by the high gradient of hydrostatic stress and large plastic deformation in this region. The resultant hydrogen concentration elicits an interplay of HELP and HEDE effects and reduces the specimen’s load carrying capacity. With properly chosen model parameters, the numerical model has the potential of serving as tool for predicting crack propagation and ductile to brittle transition due to the presence of hydrogen.