SENT testing has become increasingly important in the characterization of the base, girth weld, and HAZ structural integrity for pipelines that are operational in the low temperature regions, such as the arctic. While the SENB and CT specimens have been used traditionally in the fracture toughness characterization and assessment of pipeline materials and welds, the SENT specimen is better representative of the constraint behavior of surface cracks found in service. Further, the SENT specimen is closer in representing the ductile-to-brittle transition temperature region for the pipe operation.
For strain-based design, ECA, and stress-based fracture analyses of girth weld defects, SENT test results have been found to be more representative of the constraint of a surface crack in a pipe and hence better reflects the material toughness. Typically the CTOD as a function of crack growth is used in girth weld defect analyses, but J-R curves can be calculated at the same time. While several procedures for SENT testing (DNV, Exxon Mobil, CANMET) are currently used in the industry and are based on reliable constitutive behavior, plasticity, and fracture theories and experimental methods, more recently a British Standard (BS8571:2014) has been published. In these procedures, either the d-c EP method or the unloading compliance technique are used to determine the start of ductile tearing and crack growth to arrive at fracture resistance CTOD-R and J-R curves. This paper presents results from comparisons of crack growth predictions where in both techniques were used simultaneously when conducting the SENT tests. Other unique aspects of the comparative methodologies, pros and cons of each of the two methods, guidelines for fracture resistance curve development from SENT testing, and its impact on girth weld, HAZ testing are also discussed.
