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To accurately characterize plastic yield behavior of metals in multiaxial stress states, a new yield theory, i.e., the average shear stress yield (ASSY) theory, is proposed in reference to the classical Tresca and von Mises yield theories for isotropic hardening materials. Based on the ASSY theory, a theoretical solution for predicting the burst pressure of pipelines is obtained as a function of pipe diameter, wall thickness, material hardening exponent, and ultimate tensile strength. This solution is then validated by experimental data for various pipeline steels. According to the ASSY yield theory, four failure criteria are developed for predicting the burst pressure of pipes by the use of commercial finite element softwares such as ABAQUS and ANSYS, where the von Mises yield theory and the associated flow rule are adopted as the classical metal plasticity model for isotropic hardening materials. These failure criteria include the von Mises equivalent stress criterion, the maximum principal stress criterion, the von Mises equivalent strain criterion, and the maximum tensile strain criterion. Applications demonstrate that the proposed failure criteria in conjunction with the ABAQUS or ANSYS numerical analysis can effectively predict the burst pressure of end-capped line pipes. [DOI: 10.1115/1.2767352] Keywords: von Mises theory, Tresca theory, ASSY theory, ABAQUS, ANSYS, burst pressure, pipeline

Fracture properties of an API X80 pipeline steel have been developed using a set of single edge notched bend (SENB) and single edge notched tension (SENT) specimens with shallow and deep cracks to generate different crack-tip constraint levels. The test data show that the J-R curves for the X80 pipeline steel are strongly constraint dependent. To facilitate transfer of the experimental J-R curves to those for actual cracked components, like flawed pipeline, constraint corrected J-R curves are developed. The two-parameter J-[A.sub.2] formulation is adopted to quantify constraint effect on the crack-tip fields and the J-R curves. The constraint parameter [A.sub.2] is extracted by matching the J-[A.sub.2] solution with finite element results for a specific crack configuration. A constraint corrected J-R curve is then formulated as a function of the constraint parameter [A.sub.2] and crack extension [DELTA]a. A general method and procedure to transfer the experimental J-R curves from laboratory to actual cracked components are proposed. Using the test data of J-R curves for the SENB specimens, a mathematical expression representing a family of the J-R curves is constructed for the X80. It is shown that the predicted J-R curves developed in this paper agree well with experimental data for both SENB and SENT specimens. To demonstrate its application in assessing flaw instability, a pipeline with an axial surface crack is considered. For a crack depth of 50% of the wall thickness, the predicted J-R curve is found to be higher than that for the SENB specimen with the same crack length to width ratio. From this predicted J-R curve and crack driving force obtained by finite element analysis, the failure pressures of the pipeline at the crack initiation and instability are determined and discussed. [DOI: 10.1115/1.2349571] Keywords: crack-tip constraint, J-R curve, SENB, SENT, X80, J-[A.sub.2] solution

This paper investigates plastic collapse failure behavior and analytical assessment methods for unequal wall transition joints in transmission pipelines. The objective is to (i) validate the plastic-collapse-based code requirements that were determined by the early lower-strength pipes and (ii) develop an effective method for assessing plastic collapse failure of unequal wall joints involving modern high-strength pipes. Detailed finite element analysis was conducted to evaluate the failure behavior of transition joints and the effects of geometry, including weld taper angle, mismatched diameter and location, and material parameters, including the steel grade, mechanical property, yield-to-tensile strength (Y/T) ratio, and anisotropy. Numerical results show that the wall-thickness mismatch and tensile-strength mismatch are the two first-order parameters that control the plastic collapse failure behavior of unequal wall transition joints. Based on these first-order parameters, an analytic solution is formulated to predict burst pressure at plastic collapse as a function of the pipe geometry, material tensile and hardening properties for both end-opened and end-capped pipes in reference to the plastic instability and finite strain theory. A plastic collapse criterion and the corresponding plastic collapse assessment diagram (PCAD) are then developed as a function of the wall-thickness mismatch and tensile-strength mismatch conditions to ensure that plastic collapse failure would occur in the thinner wall with higher strength pipe. General procedures to use PCAD for assessing the plastic collapse failure of unequal wall joints are outlined. Application of PCAD indicates that high-strength pipeline grades with high Y/T ratios can be safely used beyond current code limitations on the wall-thickness mismatch of transition joints for a wide range of strength mismatch. Keywords: Plastic Collapse, Plastic Failure, Pipeline, Weld Joint, Unequal Wall, Integrity Assessment

This paper investigates the influence of yield-to-tensile strength ratio (Y/T) on failure pressure of pipelines without and with corrosion defects. Based on deformation instability and finite strain theory, a plastic collapse model for end-capped defect-free pipes is developed. The stress-strain response of materials is characterized by a power-law hardening curve, and the plastic deformation obeys the von Mises yield criterion and the deformation theory of plasticity. Two formulas to estimate the strain hardening exponent n for a specific Y/T are obtained, and a closed-form solution to the limit pressure of pipes is derived as a function of Y/T. This plastic collapse model is then extended to predict the failure pressure of pipelines with corrosion defects. Numerical and experimental comparisons are presented that validate the present models which characterize the influence of Y/T on the failure behavior of pipeline.