13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- upper surface (Figure 2.a) appears to be relatively isotropic when compared to the pearlite bands in inner the surface (Figure 2.b). However, Figure 2 shows pearlite bands in different sections of the API 5L X52 specimen. Table 1. Chemical composition of API X52 steel (weight %). C Mn P Si Cr Ni Mo S Cu Ti Nb Al 0.22 1.220 - 0.240 0.16 0.14 0.06 0.036 0.19 0.04 <0.05 .032 Table 2. Mechanical properties of API X52. E, GPA Yσ, MPa uσ, MPa A, % n K KIc MPa m 210 410 528 32 0.164 876 116.6 Figure 1. stress strain curves of X52 pipe steel with and without hydrogen absorption. Figure 2. X52 metallographic sections showing ferrite-pearlite microstructure with bands (nital etching- originally taken at (a) 100x and (b) 500x) 3. Influence of Hydrogen on Master Failure Curve Due to the assumptions, that pipes failure emanates mainly from external interferences i.e. that defect promoting failure are considered as notch, the notch fracture toughness (critical notch stress intensity factor) c K ,ρ and the critical effective T-stress ( ef c T , ) were employed to describe the material failure curve ( ) ef c c K f T , , = ρ . The critical notch stress intensity factor c K ,ρ is determined by the Volumetric Method (VM) [32]. Averaging the T-stress distribution inside the effective distance (determined by Volumetric Method), the effective T-stress (Tef) can be defined in the following
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