Dynamic Behaviour of High Strength Pipeline Steel

The occurrence of a longitudinal crack propagating along a gas pipeline is a catastrophic event, which involves both economic losses and environmental damage. Hence, the fracture propagation control is essential to ensure pipeline integrity. The commonly used ductile fracture control strategy for the design of high pressure pipelines is the Battelle Two Curve Method. This approach stipulates that if there is a crack speed at a given pressure that exceeds the gas decompression velocity at the same pressure, propagation will occur. However, for high strength pipeline steels, this method does not yield conservative predictions, as the absorbed impact energy during a Charpy test no longer reflects the actual burst behaviour of the pipe. Enhanced toughness measures, like Crack Tip Opening Angle and instrumented Battelle Drop Weight Tear test are being proposed as alternative options. These emerging toughness tests are complemented by numerical simulations of ductile crack propagation and arrest. Most of these models are based on the computation of void growth, and account for the local softening of the material due to void growth and subsequent coalescence. The constitutive behaviour of the sound pipeline steel is often modelled as merely an elastoplastic law, measured under quasi-static conditions. However, both Charpy tests and Battelle tests are dynamic events, which require knowledge of the strain rate sensitivity of the pipeline material. In addition, very high strain rates can occur in the vicinity of a running crack in a high pressure gas pipeline. Hence, the constitutive model for the pipeline steel has to account for strain rate sensitivity. In this paper, Split Hopkinson Tensile Bar (SHTB) experiments are reported on high strength pipeline steel. Notched tensile tests are performed at high strain rates, to assess the influence of both strain rate sensitivity and triaxiality on the response of the material. In addition, dynamic experiments are conducted at low temperatures (−70°C) to evaluate the ductility of pipeline steel under such severe conditions. The results allow discriminating between the effects of strain rate, triaxiality and temperature, and provide reliable experimental data to accurately model the constitutive behaviour of high strength pipeline steel.