Hydrogen Embrittlement of Steel

A stressed steel specimen with a starter notch, rendered as a real 3D lattice of iron atoms with mobile hydrogen atoms diffusing through it. Hydrogen is drawn down the hydrostatic-stress gradient toward the crack tip exactly as Oriani equilibrium predicts, and once trapped there in sufficient concentration it lowers the threshold stress intensity for crack growth (hydrogen-enhanced decohehsion), driving subcritical crack advance that can end in fracture well below the steel's dry design strength. Adjust applied stress, environmental hydrogen level, temperature and steel grade and watch the crack-tip hydrogen concentration, stress intensity factor and threshold race toward — or away from — failure.