Application of Molecular Dynamics Calculations to Elucidation of the Mechanism of Hydrogen-Induced Crack Initiation in Fracture Toughness Tests Using Tempered Martensitic Steels

: It is well known that the presence of hydrogen deteriorates mechanical properties of steels, that appears as reduced fracture toughness, shorter fatigue lifetime, etc.; these phenomena are recognized as hydrogen embrittlement . The effect of hydrogen on crack initiation of fracture toughness test has been investigated using a combination of experimental and computational approaches. Tempered lath martensitic steel was subjected to fracture toughness test with monotonically rising load in air and high-pressure hydrogen gas. While crack propagated continuously within grains in air, cracking in hydrogen grew by linking isolated interface failure ahead of a main crack tip. Then, to understand the nucleation mechanism of isolated failure in the presence of hydrogen, the tensile simulations of twist grain boundaries (TGBs) rotated along <110> axis at various angles were conducted using molecular dynamics calculations. While the dislocation emission from TGB rotated 70° is dominant deformation mode in the absence of hydrogen, the rupture along TGB rotated 110° and 170° without stress relaxation due to dislocation emission is dominant deformation mode in the presence of hydrogen. As a consequence, it is indicated that the origin of hydrogen-induced isolated crack initiation in the vicinity of fatigue pre-crack is the rupture along the block boundaries within martensitic structure due to hydrogen-induced inhibition of dislocation emission from GBs.