Probabilistic and Statistical Evaluation of Delayed Fracture Characteristics Obtained by CSRT Method

Abstract

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.

Comparison of Constant Load, SSRT and CSRT Methods for Hydrogen Embrittlement Evaluation Using Round Bar Specimens of High Strength Steels

“…The TDA results of Steel S11 with cathodic electrolysis hydrogen charging showed that the peak temperature of the hydrogen desorption curve was about 100°C, which was equivalent to that in previous studies 13,14) on similar SCM steels with tensile strengths of 1 300 MPa and 1 500 MPa. As shown in Fig.…”

“…It has been reported by Hagihara et al 13,14) that SSRT and CSRT presented almost the same relationship between the local stress and the local diffusible hydrogen concentration at the initiation site for 1 300-1 500 MPa grade steels. From this logic, the local diffusible hydrogen concentration H* accumulated by stress-induced diffusion in this study was calculated using Eq.…”

“…Recently, susceptibility to delayed fracture is evaluated by Slow Strain Rate Test (SSRT) [6][7][8][9][10][11][12] and Conventional Strain Rate Test (CSRT) [13][14][15][16] which can complete the delayed fracture test in a shorter time than CLT. The relationship between H C and fracture stress can be obtained by SSRT and CSRT.…”

“…In contrast, CSRT [13][14][15][16] is performed under a high strain rate similar to ordinary tensile testing rate which makes the stress-induced diffusion negligible. Therefore, hydrogen content at the crack initiation site of hydrogen embrittlement in the vicinity of the notch root is equivalent to the average hydrogen content in the specimen.…”

Hydrogen Embrittlement Resistance Evaluation of Ultra High Strength Steel Sheets for Automobiles

“…The Constant Loading Test (CLT), 22,23) Slow Strain Rate Technique (SSRT) 24) and Conventional Strain Rate Test (CSRT), [25][26][27] which are major evaluation methods in the field of UHSS bolts, have been minor methods in the field of UHSS sheets. Presumably, these methods are not widely used with UHSS sheets because only uniaxial deformation test can be conducted by those methods and it is more difficult to predict the risk of hydrogen embrittlement fracture in actual parts with considering the influence of forming modes.…”