Session: 03-03-01 Assessment Model Enhancements - Cracking

Paper Number: 86856

86856 - An Evaluation of the Accuracy of the Strain-Hardening Exponent Formula in Corlas Model for Cracked Pipelines 

Underground steel pipelines may experience failure due to the occurrence of cracks or crack-like anomalies as a result of internal and external factors such as manufacturing imperfection and geotechnical movement. An accurate estimation of failure pressure is essential for the pipeline integrity assessment program. For pipelines containing longitudinally cracks, previous studies have shown that failure pressures can be accurately predicted using the extended finite element method (XFEM) implemented in Abaqus. In the XFEM analysis, the actual stress-strain curve of pipeline steel is used without the need for any approximation. Alternatively, the failure pressure for longitudinally cracked pipelines can be predicted using crack assessment models such as CorLAS^TM. In CorLAS^TM, the stress-strain behavior of pipeline steel is modeled using the Ramberg-Osgood equation. However, our previous study has shown that the model error associated with CorLAS^TM (Version 2) is high, meaning that a relatively high factor of safety would be required to account for the uncertainty associated with the model. One of the sources of error could be due to the inherent deficiencies of the R-O model as it can be inaccurate beyond a limiting strain range. Another source of error could be due to the fact that the strain-hardening exponent, n, in the CorLAS^TM model is derived from the ratio of yield strength to ultimate tensile strength while neglecting the evolution of strain-hardening. By contrast, the n value in mathematical models such as the Holloman equation, Swift equation, Ludwik equation, and a more recently Ndubuaku model developed at the University of Alberta is obtained by a nonlinear curve fitting to the experimental true stress versus true strain data. Therefore, a sensitivity study is conducted to investigate the effect of n on the failure pressure predictions by using our developed XFEM models with calibrated damage parameters. The aim of this paper is to provide a state-of-the-art review on hardening formulas, explore the applicability and accuracy of the existing stress-strain models by fitting to published experimental data, and evaluate the accuracy of the hardening exponent expression in CorLAS^TM (Version 2).

Presenting Author: Meng Lin University of Alberta

Presenting Author Biography: N/A