Effects of High Strain Rates on Ductile Slant Fracture Behaviour of Pipeline Steel: Experiments and Modelling

Good material properties are required to ensure the safe and reliable design of oil and gas transmission pipelines. The main objective of the study, presented in this paper, is to examine the influence of high strain rates on the hardening and ductile fracture behaviour of an API 5L X70 pipeline steel by means of a combined experimental/numerical approach. For this purpose, the impact toughness of the material is assessed using instrumented Charpy V-notch (CVN) impact tests at a wide range of temperatures. To characterize the mechanical response of an X70 pipeline steel subjected to high strain rates, split Hopkinson tensile bar (SHTB) experiments are performed. These experiments allow deriving the true effective stress versus plastic strain, strain rate and temperature. Both the CVN and SHTB tests results are used for fundamental material research and constitutive material modelling. For the numerical simulations, the modified Bai-Wierzbicki (MBW) model is applied. The MBW model represents the influence of the stress state on the plastic behaviour and the onset of damage, and quantifies the microstructure degradation using a dissipation-energy based damage evolution law. The model hence allows for an accurate prediction of the ductile fracture mechanisms. The combined experimental/numerical approach is then used to simulate the upper shelf ductile fracture behaviour of an API X70 pipeline steel for high strain rate and Charpy tests. Based on the available experimental data, a new parameter set has been determined. Using these new material parameters, good correlations between numerical simulations and experimental observations have been obtained for both the split Hopkinson tensile bar tests and the Charpy impact tests.