• Transactions of the Korean Society of Mechanical Engineers A
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• v.29 no.12 s.243
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• pp.1621-1628
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• 2005

This paper presents a stress-modified fracture strain for API X65 steel used for gas pipeline, as a function of stress triaxiality. To determine the stress-modified fracture strain, tension test of bars with four different notch radii, made of API X65 steel, is firstly performed, from which true fracture strains are determined as a function of notch radius. Then detailed elastic-plastic, large strain finite element (FE) analyses are performed to estimate variations of stress triaxiality in the notched bars with load. Combining experimental with FE results provides the true fracture strain as a function of stress triaxiality, which is regarded as a criterion of ductile fracture. Application of the developed stress-modified fracture strain to failure prediction of gas pipes made of API X65 steel with various types of defects is discussed.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.30 no.9 s.252
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• pp.1086-1093
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• 2006

A local failure criterion for the API X65 steel is applied to predict ductile failure of full-scale API X65 pipes with simulated corrosion and gouge defects under internal pressure. The local failure criterion is the stress-modified fracture strain for the API X65 steel as a function of the stress triaxiality (defined by the ratio of the hydrostatic stress to the effective stress). Based on detailed FE analyses with the proposed local failure criteria, burst pressures of defective pipes are estimated and compared with experimental data. The predicted burst pressures are in good agreement with experimental data. Noting that an assessment equation against the gouge defect is not yet available, parametric study is performed, from which a simple equation is proposed to predict burst pressure fur API X65 pipes with gouge defects.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2012.05a
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• pp.851-852
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• 2012

• Proceedings of the KSME Conference
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• 2007.05a
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• pp.1696-1701
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• 2007

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, from which the parameters in the GTN model are calibrated. As application, the developed GTN model is applied to simulate small-sized, single-edge-cracked tensile and bend bars, via three-dimensional FE damage analyses. Comparison of FE damage analysis results with experimental test data shows overall good agreements.

• Proceedings of the KWS Conference
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• 2000.04a
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• pp.320-322
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• 2000

• Journal of the Korean Institute of Gas
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• v.16 no.3
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• pp.54-64
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• 2012

DWTT (Drop Weigh Tearing Test) is one critical method that can exhibit the fracture properties of line pipe steel, since it estimates the properties with real pipe steel. In this study, the ductile portion, inverse fracture ratio and absorbed energy of API X65 and X70 line pipe steels were estimated with temperature variation. Both steels showed that the ratio of ductile area and absorbed energy were decreased with respect to decreasing the test temperature. However, while the ductile fracture behavior exhibited until $-40^{\circ}C$ for the X70 steel, but it showed until $-30^{\circ}C$ for the X65 steel. The fracture properties were discussed with respect to test temperatures.

• Steel and Composite Structures
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• v.38 no.2
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• pp.151-164
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• 2021

During the pipe forming process, a steel plate undergoes inelastic behavior multiple times under a load condition repeating tension and compression in the circumferential direction. It derives local reduction or increase of yield strength within the thickness of steel pipes by the plastic hardening and Bauschinger effect. In this study, a combined hardening model is proposed to effectively predict variations of yield strength in the circumferential direction of API-X65 and X70 steel pipes with relatively low t/D ratio during the forming process, which is expected to experience accumulated plastic strain of 2~3%, the typical Lüder band range in a low-carbon steel. Cyclic tensile tests of API-X65 and X70 steels were performed, and the parameters of the proposed model for the steels were calibrated using the test results. Bending-flattening tests to simulate repeated tension and compression during pipe forming were followed for API-X65 and X70 steels, and the results were compared with those by the proposed model and Zou et al. (2016), in order to verify the process of material model calibration based on tension-compression cyclic test, and the accuracy of the proposed model. Finally, parametric analysis for the yield strength of the steel plate in the circumferential direction of UOE pipe was conducted to investigate the effects of t/D and expansion ratios after O-forming on the yield strength. The results confirmed that the model by Zou et al. (2016) underestimated the yield strength of steel pipe with relatively low t/D ratio, and the parametric analysis showed that the t/D and expansion ratio have a significant impact on the strength of steel pipe.

• Proceedings of the KWS Conference
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• 2001.05a
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• pp.146-149
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• 2001

• Journal of Energy Engineering
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• v.18 no.1
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• pp.49-55
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• 2009

Occurrence of hydrogen embrittlement could be one of the main obstacles for using structural equipment under hydrogen environment. It is required to develop assessment methods of hydrogen embrittlement for the metals used in production, storage, transmission and application utilities of hydrogen. The most probable method of hydrogen mass transmission is using existing natural gas pipeline. Base or weld part of the pipeline can be damaged by mixed gas of hydrogen in the pipeline. In this study small punch (SP) testing was employed to evaluate the hydrogen embrittlement behavior for a line pipe steel (API X65) with electrochemically hydrogen charged specimens. Results showed that the SP test can be a good candidate test method for hydrogen damage evaluation method. Strength of steel is known to be decreased with the level of hydrogen charging. However, for API X65 steel base metal need in this study, the effect of hydrogen to strength was not significant. It can be negligible regardless of the hydrogen contents in the steel. With this test different strength levels with various hydrogen charging conditions were observed. It can also be anticipated that more sensitive evaluation of material behavior be obtainable by the SP test method.

• Proceedings of the Korea Committee for Ocean Resources and Engineering Conference
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• 2004.05a
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• pp.401-407
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• 2004

This study is on the constant-current stress corrosion test related to the load stress in welded zone and non-welded zone of high tensile strength steel for natural gas transmission. The surface corrosion pattern of the welded zone of API 5L-X65 specimens for natural gas transmission showed global corrosion and narrow pitting, and the pitting was increased by increasing the load stress. Initially, the average relative electrode potential and the average relative current of the high tensile strength steel for natural gas transmission specimens was decreased suddenly, and the average relative electrode potential was higher and the average relative current was lower in welded zone than base metal. and the average relative electrode potential was decreased by increasing the load stress, and the average relative current was somewhat increased by increasing the load stress. The corrosion rate was less in welded zone than base metal, and the corrosion rate was decreased by increasing the load stress.