Evaluation of delayed fracture characteristics of high strength steel based on CSRT method

A conventional strain rate technique (CSRT) to evaluate the delayed fracture characteristics of high strength steels has been proposed. The critical "maximum stress-diffusible hydrogen concentration" at the delayed fracture initiation point near the notch tip is thought to be a material constant, which was originally demonstrated using the SSRT (slow strain rate technique) test method. The SSRT method takes hours to complete the test and uses a special test machine, which causes difficulty and complication. Therefore, a simple and conventional test technique, CSRT test method for delayed fracture was investigated. The crosshead speed is around 1 mm/min, so that the stress induced diffusion of hydrogen is negligible. The results obtained are as follows. (1) Since the stress induced hydrogen diffusion does not take place during the CSRT test, it is necessary to introduce the amount of hydrogen in the specimen, corresponding to the accumulated hydrogen at the vicinity of the notch tip region in the SSRT test. The electrochemical hydrogen charging conditions were established to introduce a wide range of hydrogen contents into the specimens. (2) A unique relationship between the maximum stress at the vicinity of the notch tip and hydrogen contents was obtained irrespective of the notch configuration using the CSRT test and FEM stress analysis. Therefore, it can be said that this relation is the material constants for delayed fracture.