Issue 49

B. G. N. Muthanna et alii, Frattura ed Integrità Strutturale, 49 (2019) 463-477; DOI: 10.3221/IGF-ESIS.49.44 465 Figure 1 : API X52 pipeline steel (a) an example of real corroded steel (b) Geometrical model of the pipe elbow. C Si Mn P S Cr Ni Mo Al 0.204 0.342 1.79 0023 0.0083 0.0474 0.0367 0.0067 0.0335 Co Cu Nb Ti V W Pb B Fe 0.0030 0.0648 0.0024 0.0033 0. 0041 <0.0050 <0.0010 0. 0055 97.4 Table 1: Chemical composition of the elbow steel. Yield Strength (MPa) Ultimate Strength (MPa) Elongation at failure A% Fracture toughness K IC (MPa √m ) 410 528 32 116.6 Table 2: Tensile properties of API 5L X52 pipe elbow steel. Gas flow (millions m 3 /day) Operating pressure (MPa) Operating temperature (°C) Density 2.6 – 3 7 52 0.806 Table3: Parameters of natural gas flow. R ESULTS AND D ISCUSSIONS ue to the complexities of carrying out an experimental study, we have used a numerical software in order to understand the interaction between elbow and natural gas. In particular, we have used ANSYS Workbench software with the computational fluid dynamics Software (CFD section). A steady state gas flow analysis and pressure distributions were carried out. These pressures across the elbow were obtained by the importation of the other section called APDL (Ansys Parametric Design Language) where the Fig. 2 illustrates the methodology of numerical steps. First of all, elbow radius was applied progressively in order to show the stress. At stabilization, the study of stress distributions along elbow curvature under gas pressure of 7MPa was conducted in order to get the critical position defined as the position of maximum stress. At this critical position, a semi-elliptical crack was introduced and its orientation  effect was studied in order to get the critical crack angle. This will create different semi-elliptical crack depths (a/t) in the range of 0.1 to 0.8; where “a” is the crack depth and “t” is the pipe elbow thickness. Failure assessment diagram (FAD) curve was used to evaluate the critical crack depth ratios at a critical position and a critical angle. D