Small crack effect on threshold stress intensity K_TH for high strength steel with internal hydrogen

The size effect of the threshold stress intensity K_TH for hydrogen-precharged specimens of SAE52100 were investigated. Four types of tensile specimens were used: (i) smooth specimens and (ii) specimens having various shapes of artificial defects with sizes of about 35 ∼ 500 um. In the smooth specimens, fracture origins were nonmetallic inclusions with the size of 10 ∼ 30 um (e.g. Al₂O₃·(CaO)_x, TiN and TiC). The fracture toughness determined for the small defects at fracture origin, i.e. the K_TH calculated from the defect size √area and the fracture tensile stress σ_f, showed a defect size dependence, where, area denotes the area of the domain defined by projecting the defect on a plane normal to the cylindrical axis of the specimen. The values of K_TH for both the nonmetallic inclusions and artificial defects were much smaller than those for large cracks measured by the standard WOL and CT specimens.