Influence of the Yield-to-Tensile Strength Ratio on the Failure Assessment of Corroded Pipelines

It is well known that both the yield strength and the tensile strength of a material have significant effect on the failure behavior of pipelines. Thus it can be anticipated that the yield-to-tensile strength (Y/T) is closely related to the strain hardening behavior of the material, and it also influences the failure behavior. This paper theoretically explores the influence of T/Y (the inverse of Y/T) ratio on the failure pressure of pipelines with or without corrosion defects. Based on the instability of deformation and finite strain theory, a plastic collapse criterion for close-ended pipes without corrosion defects is developed first. The constitutive behavior of line pipes is characterized by a power-law strain hardening relation, while the plastic deformation obeys the Mises yield criterion and the associated deformation theory of plasticity. An approximate relationship between the T/Y ratio and the strain hardening exponent n is obtained, and a closed-form solution to the limit pressure of pipe based on T/Y is derived. This plastic instability model is extended to predict the failure pressure of pipelines with corrosion defects, and validated by the PCRI experimental database. The results show that (a) the T/Y ratio is simply proportional to the strain hardening exponent n, which is almost independent of the yield strain and affected by the definition of the yield stress; (b) the failure pressure predicted by the present plastic instability model increases as the T/Y ratio decreases; and (c) as T/Y → 1, the present solution approaches to that predicted by the Mises strength criterion based on the nominal hoop and axial stresses.