Papers by Author: Young Pyo Kim

Abstract: Mechanical properties of the pre-strained material are different with those of virgin material without pre-strain. Buried pipelines for natural gas transmission may be deformed by outside force such as ground subsidence, ground liquefaction, cold bending and mechanical damage. Plastic deformation affects the tensile properties and fracture toughness. The effects of prestrain on the mechanical properties of API 5L X65 pipe were diversely investigated through the tensile test, crack tip opening displacement test and Charpy impact test. Axial tensile pre-strain of 1.5, 5 and 10% was applied to plate-type tensile specimens cut from the pipe body prior to mechanical testing. Tensile test revealed that yield strength and tensile strength were increased with increasing tensile pre-strain. However, Fracture toughness for crack initiation decreased with increasing tensile pre-strain. Structural integrity evaluation of the API 5L X65 pipe with crack-like flaws was assessed by using the level 2 in the API 579 code.

Abstract: A clear understanding of fatigue properties for the pipeline steel and its weld is important to provide information for pipeline design during pipeline construction and predict pipeline fatigue life during pipeline operation. The materials used in this study are API 5L X65 pipeline steel generally used for natural gas transmission. This pipeline was welded by gas tungsten arc welding (GTAW) and shielded metal arc welding (SMAW) with V-groove configuration. The fatigue crack growth behaviors of pipeline steel and its girth weld according to crack growth directions and stress ratios were investigated over a wide range of stress intensities in laboratory air.

Abstract: Pipelines for natural gas transmission may be subjected to plastic deformation by the outside force such as ground subsidence, ground liquefaction, cold bending and mechanical damage. Plastic deformation affects the tensile properties and fracture toughness. Tensile test, Crack tip opening displacement test and Charpy impact test were conducted on an API 5L X65 pipe to investigate the mechanical properties of pipeline subjected to plastic deformation. Axial tensile pre-strain up to approximately 10% was applied to plate-type tensile specimens cut from the API 5L X65 pipe prior to mechanical testing. Tensile test revealed that the yield strength and the tensile strength increased with increasing tensile pre-strain. However, the CTOD and Charpy impact energy values for crack initiation decreased with increasing tensile pre-strain.

Abstract: This paper presents a micro-mechanical model of ductile fracture for the API X65 steel, using the Gurson-Tvergaard-Needleman (GTN) model. Experimental tests and FE damage simulations using the GTN model are performed for smooth and notched tensile bars with three different notch radii, from which micromechanical parameters in the GTN model are calibrated. The calibrated micro-mechanical model is applied to quantify pre-strain effects on plastic deformation and fracture of the API X65 steel. Good agreements of the FE damage results with experimental data suggest confidence in the use of the proposed micro-mechanical model to simulate ductile failure of pipelines made of API X65 steels.

Abstract: This study presents a probabilistic methodology for estimation of the remaining life of pressurized pipelines containing active corrosion defects. A reliability assessment was carried out using extreme value distribution of the corroded defects instead of the already published failure pressure model like NG18 or ASME B31G. The failure probability of pipelines depends on the number of corroded defects, and it was calculated directly as the area exceeded a defined LV(Limited Value of corrosion depth). The remaining life of pressurized pipelines can also be estimated by the PDF of extreme value distribution as calculating the exceeded area with a defined failure probability.

Abstract: The failure assessment for the defects in multiple corroded region has been considered with the real-size pipeline burst tests and the associated Finite Element Method. The corroded gas pipeline made of X65 steel with various types of artificially machined pits has been used for a number of series of burst tests on gas pipeline. The failure pressures for the designed multiple corroded defect on gas pipelines have been measured by using in-situ burst testing and classified with respect to each designed defect sizes - length, width and depth - and distance between defects. The result from the experiments has been modeled and compared with Finite Element Method. As the result of FEM simulation, the failure pressure in multiple defects is lower than that in single defect, and closer to failure pressure in single defect of total length of defects in multiple defects as the distance between each defect is closer.