FRACTURE BEHAVIOR OF X65 Q&T SEAMLESS PIPELINE STEEL UNDER DIFFERENT STRAIN RATES AND STRESS STATES

Dynamic crack propagation in pressurized pipelines is usually investigated by means of lab-scale specimens due to its economic feasibility and material saving. More recently, new generation of pipeline steels have incredibly shown a combined fracture toughness and plastic strength capabilities with even more heavier wall thickness, for which current design standards and practice codes underpredict largely the actual material response under different strain rates. The Drop-Weight Tear Test (DWTT) is commonly used to characterize dynamic fracture behavior of pipeline steel and its numerical implementation with appropriate constitutive equations has become essential in the fundamental understanding of the interaction between fracture process and local stress-strain fields. In the present study, a X65 Q&T seamless pipeline steel is fully characterized under different strain rate levels and stress states for dynamic fracture initiation. A rate dependent phenomenological fracture criterion is proposed in the form of Modified Mohr-Coulomb (MMC) fracture model coupling with a multiplicative decomposition of the hardening law to describe strain rate effect on post-necking behavior. The model implementation is then validated through a drop-weight tearing analysis on standard and non-standard specimen configurations including different wall thickness.