Drop Weight Tear Testing of High Toughness Pipeline Material

Drop Weight Tear Testing is a common test method for determining a material’s ability to arrest a propagating crack. This testing method was developed by Battelle Memorial Institute, and is conducted in accordance with standards as API RP 5L3 ‘Recommended Practice for Conducting Drop Weight Tear testing on Line Pipe’ and EN 10274 ‘Metallic Materials - Drop Weight Tear Test’. One problem that has been encountered when performing Drop Weight Tear Testing of high toughness TMCP materials is that the pre-deformed material in the pressed notch is not sufficiently embrittled to ensure initiation of a brittle fracture. According to prevailing standards a brittle initiation is necessary for a valid test result. The material opposite the notched side (impact side) will deform quite considerably and is due to strain hardening expected to loose toughness prior to the actual fracture initiation takes place. Consequently high toughness material may give poor test results. In that respect, DNV initiated a Joint Industry Project called ‘Drop Weight Tear Testing of High Toughness Pipeline Material’, where the main objective was to obtain a better understanding on how the results from the DWTT should be interpreted for high toughness pipeline steels. During the project an extensive amount of Drop Weight Tear tests (DWTT) were performed on relevant modern pipeline steels. The resulting shear ratios were determined according to conventional fracture surface evaluation methods as well as newly developed methods as presented in the literature. The appearance of energy curves for both regular DWTT specimens and specimens with varying back gouge depths was also considered in the investigation and the consistency between the estimated shear ratios and the corresponding measured absorbed energies were thoroughly evaluated. This paper summarizes the results and recommendations obtained in the performed investigations.