13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- The effect of T-stress on crack paths has been investigated for various specimen configurations and materials [17-19]. But some results of the crack path estimation give ambiguous data; for example, it was shown [20] that if the T-stress in front of a flat crack is negative then the crack grows along the crack plane. In the case of a positive T-stress, the crack deviates from its initial plane. It was also observed that the crack did not turn immediately when the T-stress became positive but at a considerably higher value. The method employed in Ref. [21,22] was developed for a kink which is formed at a given angle to the main crack [23]. Actual attention has been paid to problem of hydrogen pipeline systems due to strong world asking in energy and environmental problems. One future way is to use hydrogen as energy vector. In one European project [24], hydrogen transport will be provided way by adding hydrogen to natural gas in existing networks. Experience shows that the majority of pipeline failure initiates from defects or cracks [25-27]. Effects of transported hydrogen affect material mechanical properties, namely, hydrogen embrittlement [28,29]. The external environmental conditions cause free corroding processes, where hydrogen is product on metal surface as result of cathodic counterpart of the anodic dissolution reaction [30,31]. The gas pipeline industry recognizes this well as a major problem. The aim of present work is to study the influence of hydrogen coupled with constraint (T stress is used as constraint parameter) on master failure curve and crack path direction and stability. In the first part the influence of hydrogen on master failure curve determined from fracture tests performed on different specimens geometries (CT, SENT, RT and DCB) has been studied. In a second part, fracture path under low constraint (negative T-stress) has been studied. Fracture has been obtained by burst tests under hydrogen pressure of pipes. In the third part, fracture path under high constraint (positive T-stress) has been studied from fracture of DCB specimens. Finally, a proposed mechanism of crack extension with constraint and hydrogen embrittlement has been proposed. 2. Material The material used in this study is an X52 steel meeting requirements of API 5L standard. API X52 steel was the most common gas pipeline material for transmission of oil and gas during 1950-1960. Chemical composition of the studied steels is given in Table 1. In table 2, the mechanical properties of API X52 steel have been presented. E, Yσ, uσ, A%, n, k and KIc are the Young’s modulus, yield stress, ultimate stress, elongation at fracture, strain hardening exponent and hardening coefficient of Ramberg-Osgood law, and fracture toughness, respectively. Stress strain curves of X52 steel have been determined with and without hydrogen absorption and reported in Figure 1. Classical tensile properties such as yield stress and ultimate strength increases also when hydrogen is absorbed in steel as indicated in Table 2. A small increase of yield stress has been noted (2.5%) as an important reduction of elongation at failure (38%). Static stress-strain is obtained by fitted tensile test results using hardening power law n K ε σ . = . Microstructure of API 5L X52 steel in the longitudinal and transverse orientation is shown in Figure 2 (a) and Figure 2 (b), respectively. These pictures show the distribution of the ferrite and pearlite with respect to orientation. The distribution of pearlite in
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