Ductile and brittle fracture assessment of pipeline steels using advanced dynamic tensile tear testing methodology.

Fracture propagation control of high-pressure pipelines is an essential strategy to avoid a catastrophic event that includes both economic losses and environmental damage. To ensure structural integrity over several decades, the pipe material is subjected to an extensive fracture analysis. Lab-scale fracture testing is an essential tool to determine a materials’ resistance against crack propagation. Standardised tests such as the Charpy V-Notch (CVN) and the Drop Weight Tear Test (DWTT) are widely-known and used extensively in industry. These tests were introduced before high-grade steels were used for pipeline construction. Furthermore, the correlation between these tests and in-service pipelines are solely based on empirical relation with a database of full-scale burst tests. Consequently, there is an increasing call to characterise pipeline material with fracture tests mimicking in-service conditions. 

In this study, the Dynamic Tensile Tear Test (DT3) was investigated as an alternative lab-scale fracture test. The proposed methodology applies a dynamic tensile load to fracture a full-thickness plate-like specimen. The experimental setup was used to analyse the fracture behaviour of an X70 grade high-grade pipeline steel. A testing campaign was conducted, focussing on the characterisation of fracture behaviour in the upper shelf, lower shelf and transition region. To perform tests in these regions, certain challenges required new experimental solutions:

• Upper-shelf region: Implementation of a high-speed camera setup was required to obtain detailed information on the dynamic fracture behaviour and observe the formation of the Crack Tip Flipping (CTF) phenomenon. Through highspeed stereo Digital Image Correlation (DIC), the mechanism of CTF could be investigated.

• Lower-shelf region and transition region: A large-surface spray cooling system was created to allow testing in the lower-shelf region. Low temperatures could be achieved using liquid nitrogen as cooling substance. The cooling rate and iii iv ABSTRACT temperature uniformity were analysed before integration within the DT3 system. The integrated cooling system allowed to perform experiments at any given target temperature in the lower-shelf and transition region.

Analysis of the obtained data allows to characterise the ductile-to-brittle transition behaviour and estimate the ductile-to-brittle transition temperature (DBTT). A comparison with impact-based CVN and DWTT experiments was performed, indicating a good approximation using the new proposed methodology without requiring empirical relation or additional correction factors. To support the experimental data, all obtained fracture surfaces were investigated using 3D imaging as well as Scanning Electron Microscopy (SEM). These imaging techniques provided additional insight in the fracture behaviour as well as the formation of ArrowHead Markings (AHMs).

In complement to the experimental work, numerical models were created to validate the experimental observations. Ductile and brittle failure are considered using Finite Element (FE) analysis with implementation of advanced damage models such as the Gurson-Tvergaard-Needleman (GTN) model, and the Modified Bai-Wierzbicki (MBW) damage model. The numerical models were able to reproduce the same fracture behaviour as observed during the DT3 experiments. Finally, a new Hybrid Fluid-Structure (HFS) framework was developed to assess crack propagation and crack arrest in full-scale burst tests. 

The results in this study, allow characterisation of pipeline steels in the uppershelf region, transition region, and lower-shelf region using the DT3 system. Based on the obtained results, it has been shown that the DT3 setup, as a non-standard testing method, shows great potential to be considered as an alternative lab-scale test without the need for empirical relations. This could prove critical for future pipeline applications since the empirical relations for conventional lab-scale testing are based on a database of full-scale tests with steel grades up to X65. Consequently, the DT3 method might provide a lab-scale solution for material characterisation of high-grade pipeline steels and DBTT prediction before resorting to full-scale testing.