Effects of Prestrain on Fracture Toughness and Fatigue-Crack Growth of Line Pipe Steels

To investigate the failure mechanism of pipelines subjected to mechanical damage, Charpy impact, crack-tip-opening displacement (CTOD) and fatigue-crack growth tests were carried out for six series of line pipe steels with uniaxial plastic prestrain, ε_pr. The Charpy absorbed energy and critical CTOD (δ_c) decreased with increasing |ε_pr|; ln δ_c = α ε_pr + β. The derivative, dδ_c/dε_pr, was dependent on the ductile-to-brittle transition temperature of the steels. In the CTOD tests, the prestrain caused ductile-to-brittle transition for the steels with a higher transition temperature. The effects of the compressive ε_pr on both the reduction of δ_c and ductile-to-brittle transition were larger than those of the tensile ε_pr. The compressive ε_pr accelerated both the fatigue-crack initiation and growth.