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Mohammad Tavid, Sayyed Hashemi,
Volume 23, Issue 1 (January 2022)
Abstract
Fatigue failure is the most common type of failure in structures under oscillatory loading. Fatigue damage in steel gas pipelines is very important due to internal pressure fluctuation. A large part of pipelines in oil and gas industry of Iran are made of thermomechanical steel of grade API X65, made by spiral submerged arc welding. In this study, the stress-life curve and fatigue limit of the spiral weld seam of this steel are determined by fatigue tests. For this purpose, 20 test specimens (12 specimens used in the limited fatigue life zone and 8 specimens used to estimate fatigue strength) according to ISO 1143 standard. All test samples were cut from an actual spirally welded pipe with 1219mm outside diameter and 14.3 mm wall thickness and were tested on a completely reverse rotating-bending fatigue machine. Statistical analysis of the results was performed by considering the normal logarithmic distribution. Mean curve, confidence interval, and characteristic curve of the results were obtained in the finite fatigue life range using Basquin fatigue model according to ISO 12107 and ASTM E-739 standards. In the fatigue resistance range ISO 12107 standard was used. The mean endurance limit of the seam weld of the tested steel was 258.5 MPa which is located in the conventional range of 0.4 to 0.6 of the ultimate tensile strength of this steel.
Mohammad Tavid, Sayyed Hashemi,
Volume 24, Issue 5 (May 2024)
Abstract
Thermomechanical steels are widely used in oil and gas pipelines due to their high toughness and high resitance against crak growth. A large part of the steel pipelines used in the oil and gas industry in Iran is made of API X65 steel. The fluctuations of internal gas pressure in steel pipes can cause fatigue failure and lead to gas leakage and explosion. So, the control of damage initiation and structural integrity of gas pipelines is of great importance. In this study, the S-N curve and the fatigue strength of the base metal of the API X65 steel were estimated by performing fatigue tests. For this purpose, 24 and 25 test specimens along the seam weld in the coil transverse direction, and perpendicular to the seam weld along the coil rolling direction were prepared according to ISO 1143 standard, respectively. All test samples were cut from an spirally welded pipe with 1219mm outside diameter and 14.3mm wall thickness and were tested on a completely reverse rotating-bending fatigue machine. Statistical analysis of the results was performed by considering the normal logarithmic distribution. The mean curve, characteristic curve, and confidence interval of the results were obtained both in the finite fatigue life range and in the fatigue resistance. The mean endurance limit of the base metal perpendicular to and parallel to the seam seam were 305 and 291 MPa, respectively which were in the range of 0.4 to 0.6 of material tensile strength and above the seam weld endurance limit (258 MPa).
