The Use of Crack-Tip Opening Displacement for Testing of the Hydrogen Embrittlement of High-Strength Steels

Abstract

Stress-corrosion cracks are, as a rule, brittle and often encountered under stresses much lower than the yield strength. For this reason, the methods of linear elastic fracture mechanics (LEFM) can be used for the investigation of stress-corrosion cracking (SCC). However, in some cases, these methods are inapplicable, and it is necessary to use the methods of so-called elastoplastic fracture mechanics (EPFM). In the EPFM approach, the J-integral is the most commonly used parameter for correlating crack initiation and propagation but the crack-tip opening displacement (CTOD) and crack-tip opening angle (CTOA) prove to be promising alternatives, especially for thin-sheet materials. Since both these parameters are connected with the crack geometry and, hence, reflect the level of strain at the crack tip, they appear to be useful correlation parameters for the cases of SCC, where the level of strain in the vicinity of the crack tip and, in particular, the strain rate, are the determining variables of the process. The hydrogen embrittlement of a higher-strength structural steel and welded joints of a C-Mn steel is assessed by using the CTOA and CTOD methodologies. In constant-extension-rate tests (CERT), fatigue precracked specimens were loaded with various low strain rates and electrolytically charged with hydrogen. It was discovered that hydrogen embrittlement significantly affects the crack-growth resistance curves (R-curves) thus generated and the opening angle for which the crack propagates into the material.