A novel methodology to characterise the ductile-to-brittle transition behaviour of pipeline steels using the dynamic tensile tear test

Abstract

Brittle fracture control strategies for high-pressure pipelines are essential to prevent catastrophic failures resulting in significant economic losses and environmental damage. Typically, this is achieved by operating the pipeline well above the ductile-to-brittle transition temperature (DBTT). In general, a series of lab-scale fracture tests, such as the Charpy V-Notch (CVN) or Drop Weight Tear Test (DWTT), are performed at different temperatures to construct the ductile-to-brittle transition curve and determine the DBTT. However, when applied to high-grade steels, correction factors or additional criteria are often necessary to obtain satisfactory results. As an alternative lab-scale testing methodology to characterise high-grade steels, the Dynamic Tensile Tear Test (DT3) has been introduced. Recently, an experimental campaign was conducted with the implementation of a liquid nitrogen spray cooling system to characterise the ductile-to-brittle transition behaviour of X70 grade steel using the DT3 system. In this work, tests were performed at temperatures ranging from −80 °C to 25 °C. The transition behaviour was analysed using measured data, and fractography was performed using Scanning Electron Microscopy (SEM). The DBTT for the considered X70 grade steel was determined, and the performance of the DT3 methodology was compared to CVN and DWTT testing methods. Additionally, numerical models were created using Finite Element (FE) modelling and results were compared to the obtained experimental data. By implementing a stress-strain cleavage criterion in combination with the MBW material model, the models considered both ductile damage and failure, as well as brittle failure and mixed ductile-brittle failure. It can be concluded that the DT3-based transition curve provided a satisfactory DBTT prediction. Furthermore, numerical results showed a good correlation with experimental observations and demonstrated a similar transition behaviour.