Determination of Crack Initiation in Ultrahigh Strength Martensitic Steel by Interrupted Tensile Test

The study investigated crack initiation in ultrahigh strength martensitic steel (1926 MPa tensile strength, >13% elongation) using interrupted tensile tests and hydrogen charging. The steel's ultrafine microstructure (5 μm prior austenite grains, 1.3 μm martensite blocks) enabled simultaneous high strength and ductility. Fracture transitioned from microvoid coalescence to localized quasicleavage after hydrogen charged. Crack initiation occurred at 10.3%–11.8% engineering strain, corresponding to true strains of 0.42–0.54 in necking regions by simulation. High‐density boundaries effectively inhibited crack propagation during damage evolution, contributing to exceptional nonuniform elongation.