Prestrain history effect of ductile fracture of pipeline steels

Before or during operation, pipelines may suffer from plastic pre-deformation due to accidental loading, cold bending or ground movement. One of the examples is the so-called pipe reeling process. In order to ensure effective and safe pipeline transports, it is important to evaluate the pre-deformation (pre-strain) induced pipe properties changes and include their effects into lifetime design and structural integrity assessment. In this study, single edge notched tension (SENT) specimen was used to simulate the effect of a plastic pre-straining cycle on the crack tip driving force and near tip stress fields. The so-called complete Gurson model developed by the authors has been applied to simulate the ductile fracture behaviour. It has been shown that plastic prestrain cycles will alter the relation between the crack tip driving force and crack tip stress fields – the crack tip constraint. In general the crack tip constrain will be increased and the fracture resistance will be reduced by the prestrain history. This effect must be included in future structural integrity assessment of pipeline components.