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The present paper proposes ductile failure criteria in terms of true fracture strain (the equivalent strain to fracture) as a function of the stress triaxiality (defined by the ratio of the hydrostatic stress to the equivalent stress) for the API X65 steel. To determine the stress-modified fracture strain, smooth and notched tensile bars with four different notch radii are tested, from which true fracture strains are determined as a function of the notch radius. Then detailed elastic-plastic, large strain finite element analyses are performed to estimate variations of stress triaxiality in the tensile bars, which leads to true fracture strains as a function of the stress triaxiality, by combining them with experimental results. Two different failure criteria are proposed, one based on local stress and strain information at the site where failure initiation is likely to take place, and the other based on averaged stress and strain information over the ligament where ductile fracture is expected. As a case study, ligament failures of API X65 pipes with a gouge are predicted and compared with experimental data.

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The fatigue crack growth rate and fracture toughness tests of type 304 stainless steel were studied over a temperature range of -162 o C to room temperature. Girth weld metal specimens were fabricated using a combination of gas-tungsten-arc-welding and shielded-metal-arc-welding. The seam weld metal was made with submerged arc welding. Fatigue crack growth rate tests were conducted using compact tension specimens in accordance with ASTM E647. Fracture toughness was evaluated through CTOD tests with three point bend specimens. The CTOD values were affected by crack orientation with respect to the rolling direction, but orientation had no influence on fatigue crack growth rates. The fatigue crack growth rates and the CTOD values decreased with decreasing test temperature. INTRODUTIONThe demand for clean and convenient natural gas has been continuously increasing. Natural gas is more efficiently stored and transported as liquefied natural gas (LNG) below the liquefaction temperature because the volume of LNG is about 1/600 that of the gaseous form. The materials for storage and transmission of LNG should have good mechanical properties at low temperature because the boiling point of LNG is -162 o C under 1 atmosphere [1-3]. The structural materials for cryogenic application should exhibit high strength, ductility, fracture toughness, good weldability and fabrication [4]. Type 304 austenitic stainless steel, which has excellent low temperature performance and is stable, is the most common material for pipes used to carry LNG [2,3]. Type 304 austenitic stainless pipes for LNG transmission are invariably joined by welding and the fracture origins of the welding structures are mainly the degradation of mechanical properties due to variation in microstructure through thermal cycles during welding. The integrity of welded structures is considerably reduced by fatigue in the presence of such weld metal defects as incomplete fusion, gas pore, undercut and slag inclusion. The pipes for LNG transmission are subjected to repeated thermal stress as a result of temperature gradients and pressure fluctuation due to changes in operating conditions [2,3,5]. The coefficient of thermal expansion of 304 austenitic stainless is about one and a half times higher than that of carbon steel [6]. Stainless steel pipes for LNG transmission are subject to significant thermal contraction under cryogenic temperature. The result of this could be increased distortion, buckling and fatigue fracture at the weld joint. There have been a large number of studies of the fatigue and fracture toughness of the weld metal and base metal of austenitic stainless steel [7-10]. Little research, however, has been done on the fatigue and fracture toughness properties at LNG temperature. Mukai et al., investigated the tensile and fatigue properties of the base metal of austenitic stainless steel at temperatures between 20 o C and -162 o C [2]. The authors in ref.[2] did not deal with the fracture toughness of the base metal and the mechanical properties of the we...