Development of Failure Criteria for Predicting Tearing of Wrinkled Pipe

Onshore steel pipelines, particularly buried in cold region, often subjected to extreme geo-environmental conditions, where significant inelastic deformation may occur resulting in localized wrinkles. Under continued deformation, there is a possibility of excessive cross-sectional deformation at wrinkle locations, eventually leading to fracture or damage in the pipe wall. A recent field fracture and failed laboratory specimens under monotonic load history address the necessity of conducting a comprehensive research program to better understand this unique failure mode. Initial results have indicated that even under monotonic loading, significant strain reversals can occur at sharp fold of the wrinkle. These strain reversals were identified as one of the key factors to trigger this unique failure mechanism. This paper addresses the development of failure criteria used in the finite element (FE) model of plain pipes subjected to sustained monotonic axial and bending deformation with or without internal pressure. In conjunction with the strain reversal criterion, the critical equivalent plastic strain was used as the fracture or damage initiation limit in the numerical analyses. Results obtained from the full-scale test of an NPS16 pipe were used to calibrate the FE model. Results obtained from the numerical analyses have shown that the proposed criteria predict the onset of fracture at sharp fold of the wrinkle with reasonable accuracy.