13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- The Master Failure Curve of Pipe Steels and Crack Paths in Connection with Hydrogen Embrittlement M. Hadj Meliani1,2,*,A. Elhussein2, A. Elhoud3, Z. Azari2, Y.G. Matvienko4, G. Pluvinage2 T. Boukharouba5 1 LPTPM, FS, Hassiba BenBouali University of Chlef, 02000, Chlef, Algeria. 2 LaBPS-ENIM, Ile de saulcy 57045, Paul Verlaine university of Metz, France. 3 Sinal, Materials and metallurgical services division, Rue de Madrid, Annaba, Algeria. 4 Mechanical Engineering Research,4.M. Kharitonievsky Per., 101990 Moscow, Russia. 5 Laboratoire de Mécanique Avancée, LMA, USTHB, Algeries, Algeria. * Corresponding author: hadjmeliani@univ-metz.fr Abstract This paper provides some critical review of the history and state of two elastic fracture mechanics (K and T) and relationship to crack paths. A particular attention is given in the case of hydrogen embrittlement. A fracture toughness transferability curve (Kρc-Tef) has been established for the X52 pipe steels described by a linear relationship where Tef is the average value of T stress over the characteristic length of the fracture process. A mechanism involving influence of the ef c T , -stress on void growth for ductile failure is proposed; the effects of hydrogen on crack paths from the viewpoint of microstructural aspects are disputed. Keywords crack paths, hydrogen embrittlement, transferability curve, fracture toughness, T-stress 1. Introduction Fracture toughness is now considered as not intrinsic to material but depends on geometry, thickness, loading mode and more generally to constraint. Recent numerical and experimental studies have attempted to describe fracture in terms of two or three fracture parameters [1-3].The elastic stress fields in a region surrounding the crack tip can be characterized by the following solution [1]: 2 0( ) ( ) 2 3 A r r f T r K xi xj ij I ij + + + = π δ δ θ π σ (1) where IK is the stress intensity factor, fij(θ) is the angular function, ijδ is the symbol of Kronecker’s determinant. A polar coordinate system (r,θ) with origin at crack tip is used. Several methods have been proposed in literature to determine the T-stress for cracked specimen. The stress difference method has been proposed by Yang et al. [4]. It was noted [5-8] that T-stress characterizes the local crack tip stress field for elastic linear material, and the elastic plastic material with the restriction of small-scale yielding conditions. Various studies have shown that T-stress has significant influence on fracture toughness, crack growth direction, A3 has some influence crack stability [9-15]. The K-T and K-A3 approaches lead to a two-parameter fracture criterion. With K as the driving force and T or A3 a constraint parameters, a master failure curve can be successfully used to take into account the constraints of stress fields for various proposed geometry and loading structure configurations. Recently, the master curve has been evolved into a mature technology for characterizing the notch fracture toughness to quantify constraint effects for different testing specimen and structures [16].
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