The influence of crack speed on dynamic fracture toughness of pipeline steel has been observed in some recent tests, although it is still a challenge to obtain a specific relationship between dynamic fracture toughness and crack speed due to the expensive costs of experiments. Meanwhile, the understanding of the dependence of fracture toughness on crack speed is critical for material selection and crack-arrest design in high-strength steel pipelines. The present work develops a strain rate-dependent cohesive zone model and related finite element model to analyze speed-dependent dynamic fracture of pipeline steels observed in recent drop-weight tear tests. Different than most of existing cohesive zone models, the traction-separation law of the present model considers the role of rate of separation, and a strain rate-dependent elastic-viscoplastic constitutive model is employed for the bulk material. The speed-dependences of crack-tip-opening angle (CTOA) and energy dissipation observed in experiments are reproduced in our simulations for crack speed up to 150 m/s. A remarkable feature of the present work is that the present rate-dependent model can predict speed-dependent fracture as a consequence of the strain rate effect even when all fixed material parameters are speed-independent. These results suggest that the strain rate effect in the bulk material could be largely responsible for the speed-dependent dynamic fracture of pipeline steels, and the present rate-dependent model could be used to simulate dynamic fracture of pipeline steels especially when experiments are difficult or too expensive.

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