Effect of Prestrain on Tensile and Fracture Toughness Properties of Line Pipes

To investigate the mechanical properties of pipeline subjected to plastic deformation, tensile tests and crack-tip-opening displacement (CTOD) tests were conducted on X60, X65, and X80 line pipe steels with uniaxial tensile or compressive prestrain, $εpr$⁠. The tensile tests revealed that the Bauschinger effect and work hardening were dependent on the yield-to-tensile ratio $(Y∕T)$ of the steels; the change in yield stress due to prestrain linearly depended on the $Y∕T$ of the base metal. A critical CTOD for crack initiation decreased with increasing $∣εpr∣$⁠. Compressive prestrain had a larger effect on the reduction of the critical CTOD than tensile prestrain. The test results showed that the reduction in the critical CTOD due to tensile prestrain was predictable with a simplified theoretical model employing a critical fracture strain of the base metal.