The Use of Crack-Tip Opening Displacement for Testing of the Hydrogen Embrittlement of High-Strength Steels
- PDF / 790,908 Bytes
- 7 Pages / 612 x 792 pts (letter) Page_size
- 29 Downloads / 176 Views
THE USE OF CRACK-TIP OPENING DISPLACEMENT FOR TESTING OF THE HYDROGEN EMBRITTLEMENT OF HIGH-STRENGTH STEELS W. Dietzel 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.
Introduction Stress corrosion cracking (SCC) is a time dependent phenomenon controlled by microstructural and metallurgical features and localized electrochemical processes at the crack tip. The macromechanical approach of fracture mechanics helps to characterize the SCC behavior of materials and develop a guidance for avoiding or controlling SCC during service. In the fracture-mechanics treatment of SCC, it is normally assumed that stresscorrosion cracks are brittle and propagate in an elastic body, despite the fact that local plasticity may be necessary for the process of cracking. Hence, the linear elastic fracture mechanics (LEFM) is most extensively used for studying SCC and the crack-tip stress-intensity factor in the opening mode ( KI ) is used to represent the mechanical driving force controlling crack initiation and propagation [1–5]. By using the threshold of the stress intensity factor for the onset of SCC ( KISCC ), it is possible to predict the combinations of the stress level with flaw sizes and shapes resulting in SCC. Together with the crack growth data ( da / dt vs. KI ), KISCC yields information about the severity of environments which can promote SCC and the efficiency of countermeasures and protection means. They can be used to establish subcritical crack-growth allowables for both new designs and existing structures, i.e., to decide whether a period of safe crack growth
Data Loading...
