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 (YT) 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 YT ratios can be safely used beyond current code limitations on the wall-thickness mismatch of transition joints for a wide range of strength mismatch.

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