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, epsilon(pr). 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 vertical bar epsilon(pr)vertical bar. 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.