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Fractography of drop weight tear test (DWTT) specimens has received great attention by researchers in recent years due to the complex fracture surface of this test specimen. In this research, macroscopic characteristics of fracture surface of spiral seam weld in API X65 pipeline steel are investigated for the first time using chevron-notched DWTT specimensTest specimens were machined from an actual steel pipe of API X65 grade with an outside diameter of 1219mm and wall thickness of 14.3mm. Then chevron notch of 5.1, 10 and 15mm depth was placed in the center of each specimen and test samples were fractured under dynamic loading of 7m/s. Fractography of the fracture surface of test specimen with 5.1mm notch depth (as typical of test samples) showed that cleavage flat fracture initiated from the notch root (where stress intensity factor was high). Cleavage fracture changed immediately to ductile shear fracture, deviated to one side of specimen and grew extensively in heat affected zone, and finally terminated in base metal. Delaminations were observed in shear fracture area almost parallel to crack growth direction. After that, shear lips and inverse fracture appeared in hammer impacted area. By calculating the percent shear area from standard formulations, it was found that test specimen had above 95% shear area, and ductile fracture was the dominant fracture mode implying the fitness of tested steel for application in high-pressure gas transportation pipelines.

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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.