Sensitivity to hydrogen embrittlement of AISI 4140 steel: A numerical study on fracture toughness

Contamination of steel structures, working in environments where the presence of hydrogen cannot be neglected, can lead to significant degradation of mechanical properties, in particular, fracture toughness. In order to estimate the local hydrogen concentration at the crack tip and to understand the embrittlement mechanism, numerical models are important tools to support experimental tests that are quite complex to perform. This paper presents the application of a cohesive zone model, which couples diffusion and mechanical fields, to study the hydrogen embrittlement on AISI 4140 steel. The total hydrogen concentration, sum of the contents of hydrogen present in the lattice and in dislocation traps, is the quantity governing the embrittling effect. The input parameters of the model were calibrated using experimental tests performed on steel samples; then, initial lattice concentration was calibrated based on hydrogen pre-charged tests. A sensitivity analysis was proposed, discussing the effects of material, environmental and testing input parameters. The analysis confirms the capability of this numerical tool in predicting the mechanical response in presence of hydrogen, highlighting its potential to be used for practical design and assessment.