Atomistic study of hydrogen embrittlement during cyclic loading: Quantitative model of hydrogen accumulation effects

- Proposing a model to quantify hydrogen accumulation effects in cyclic loading.
- Acceleration factor of crack growth in the presence of hydrogen can be predicted.
- The prediction agrees well with experimental observations.
- The model bridges nano-scale simulations with field tests.

Hydrogen embrittlement (HE) is a serious industrial problem that can reduce the ductility, cause catastrophic brittle failures of susceptible materials. Although previous atomistic HE study is able to quantify hydrogen delivery to the crack tip with the stress intensity distribution, no mathematical model has been proposed to quantify the hydrogen effects in the complex loading spectra. In current study, we first employ molecular dynamics (MD) simulation to study propagation behavior of a single crack with pre-charged hydrogen under cyclic loading. The MD simulation demonstrates that hydrogen atoms will diffuse to the crack tip during loading and unloading and form hydrogen rich region. Once hydrogen-rich region forms, cracks can propagate during unloading because of the brittle nature due to hydrogen accumulation. After the crack tip passes through hydrogen-rich region, it becomes blunted, and re-starts to accumulate hydrogen atoms. In a separate simulation, we closely monitor the hydrogen accumulation during ten cyclic loads and found that hydrogen atoms accumulate linearly in a nano-size region ahead of crack tip. Based upon the MD simulation results, we proposed a new model that could be used to quantify hydrogen accumulation effects in cyclic loading, especially minor cycles. Furthermore, the proposed model was used to predict the acceleration factor for crack growth in the presence of hydrogen and it shows excellent agreement with experimental results.