Apparent fracture toughness of low-carbon steel CSN 411353 as related to stress corrosion cracks L’ubomír Gajdoš a,⇑ , Martin Šperl a , Jan Siegl b a Institute of Theoretical and Applied Mechanics – Academy of Sciences of the Czech Republic, v.v.i., Department of Thin-Walled Structures, Prosecka 76, Prague 9, 190 00, Czech Republic b Department of Materials, Czech Technical University in Prague – Faculty of Nuclear Sciences and Physical Engineering, Prague, Czech Republic article info Article history: Received 2 December 2010 Accepted 2 April 2011 Available online 9 April 2011 Keywords: E. Corrosion E. Fracture G. Fractography abstract The occurrence of cracks in structural components indicates a certain threat to their reliable operation, because these cracks can grow during operation and reach critical sizes, leading to fracture. The fracture resistance of a structural component is given by the fracture toughness of the material, determined on standardized specimens with a precycled fatigue crack, and the constraint. The fracture toughness itself depends also on the environment. There is enough evidence that in the conditions of the environment assisted cracking the fracture toughness can be significantly reduced by hydrogen mechanism. Our research results have confirmed this and have demonstrated a considerable reduction in the stress cor- rosion fracture toughness as compared to that related to fatigue cracks. This should be taken into account when assessing the integrity of structural components with stress corrosion cracks. This paper presents experimental results concerned with the stress corrosion fracture toughness of specimens from a DN150 gas line pipe made of low-C steel CSN 411353. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Stress corrosion cracking (SCC) is the most insidious type of damage to structures such as high-pressure pipelines, pressure vessels and storage tanks [1]. A good comprehensive survey of the basic concepts of this type of damage is given in the paper of Woodtli and Kieselbach [2]. The insidiousness of stress corrosion cracking lies in the fact that it often occurs without any visible deformation of the material and is very hard to be identified, i.e. timely diagnostics is very difficult [3]. It can therefore lead to unex- pected fractures. SCC may occur by several mechanisms; one of them is the hydrogen mechanism leading to hydrogen embrittle- ment (HE). In the HE process hydrogen atoms from the environ- ment enter the metal and diffuse in the lattice. They tend to be attracted to regions of high triaxial tensile stress where the metal structure is dilated. Thus, the hydrogen atoms are drawn to the re- gions ahead of crack tips or sharp notches that are under stress. They can occupy the interstitial sites in the metal lattice, as shown by neutron diffraction in many cases [4], and dilate thus the crystal lattice. Hydrogen atoms can also be trapped by dislocations and can influence their mobility [5]. It is not quite clear what is the direction of this influence; it seems, however, that it is more likely that hydrogen atoms enhance rather than retard the motion of dis- locations [6]. When H atoms are trapped by vacancies, impurities and/or grain boundaries there is enough space for the atoms to recombine to form molecules. These are more stable than are the arrangements of H atoms in the lattice but the internal pressure they are exerting in the traps generates sufficient stress to either to nucleate and propagate a crack [7] or to make the local region embrittled [8]. All these processes can lead to the reduction of duc- tility ahead of the crack tip reducing the fracture toughness of the material. SCC pertains to environmentally assisted cracking (EAC). The concepts of linear elastic fracture mechanics are commonly used in SCC to determine the susceptibility of engineering materials to SCC in various environments. The studies are mostly directed at determining the threshold value K ISCC , below which the SCC would not occur [9] or at determining the crack growth kinetics curves as a fundamental tool for design against SCC [10,11]. Elastic plastic fracture mechanics concepts are used in stress corrosion crack growth by employing the J integral as a fracture parameter to determine the onset of crack growth (J Ic ) and the J-based R curve (J–Da) [12]. A number of different testing methods are used for studying the SCC processes. They usually utilize a procedure that standard specimens are exposed to a corrosive environment and at the same time they are subjected to mechanical loading [13]. The R curves for a specific material in a certain environment are customarily obtained by the partial unloading method at moderate displacement rates or by the D.C. potential drop method preferably 0261-3069/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2011.04.001 ⇑ Corresponding author. Tel.: +420 286882121; fax: +420 286884634. E-mail address: gajdos@itam.cas.cz (L. Gajdoš). Materials and Design 32 (2011) 4348–4353 Contents lists available at ScienceDirect Materials and Design journal homepage: www.elsevier.com/locate/matdes