Constitutive relation and failure criterion for X60 pipeline steel
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Abstract:
The X60 pipeline steel is the steel which is most commonly applied to the transportation of oil and natural gas，but under the external load，deformation or stress concentration is more likely to occur on the X60 pipeline steel，and thus it results to buckling even cracking failure. For studying the mechanical behavior of the X60 pipeline steel，evaluating its mechanical safety has become one of the most important works for the pipeline risk management. Among all the works of risk management，the constitutive relation equation is the basis of the whole mechanics research，and the failure criterion determines the pipeline’s life cycle. By comparing the static tensile stress-strain relationship and the Ramberg-Osgood constitutive relation of X60 pipeline steel，we find when ε≤εP0.2，the Ramberg-Osgood constitutive equation can more accurately reflect the X60 pipeline steel’s tensile stress-strain relationship；however，when ε＞εP0.2，the theoretical constitutive curve，which is based on the experimental results of Ramberg-Osgood constitutive model，has larger deviation. Based on this observation and according to the different mechanical characteristics during the static tensile elastic stage and plastic stage，we make a revision on the Osgood constitutive model by using the interpolation method and propose the X60 pipeline steel’s global 2-phase stress strain relation under the condition of unidirectional pull-up，which can represent the X60 pipeline steel’s tensile stress-strain relationship more accurately. Considering the effects caused by the weld seam and heat-affected zone to the pipeline strength and stiffness，and combined with the failure criterion based on the stress and strain about plastic material，this article proposes the influence factor and the failure criterion of the strain control for the X60 pipeline steel，also establishes critical stress calculation equation based on the fourth strength theory at the time of pline failure.
