The delayed fracture characteristics of steels are expressed by the relationship between the maximum fracture stress and the hydrogen content at the notch tip of circumferentially notched round bar specimen, where delayed fracture initiates. This material constant is easily obtained by CSRT (Conventional Strain Rate Technique) method. The CSRT tests with the notch tip radius of 0.1, 0.25 or 0.8 mm were carried out on the high strength steel having 1300 MPa in tensile strength. Based on the probabilistic and statistical evaluation of the CSRT test results, the specimen with the notch root radius of 0.25 mm gives stable and average results of all experiments. Moreover, this notch geometry has the comparable stress concentration factor to the bottom of the actual bolt screw and easily machined with high accuracy. From these points of view the 0.25 mm radius notch is considered to become the standard specimen geometry. The scattering of delayed fracture characteristics was evaluated by applying the P–S–N method of fatigue test and the P–S–H method was demonstrated.
