Constraint-Driven Fracture Toughness Assessment of Unequal-Wall-Thickness X80-X60 Girth Welded Pipelines Using SENT Specimens

Unequal-wall-thickness X80-X60 girth welded joints used in pipeline transition sections exhibit strong local mechanical heterogeneity, while the wall-thickness transition also introduces a separate structural geometry effect. Conventional homogeneous or weld-metal-only descriptions may therefore be insufficient for interpreting the fracture response of single-edge-notched tension (SENT) specimens sampled from such joints. In this study, a constraint-driven fracture toughness assessment was conducted for single-edge-notched tension (SENT) specimens representing the X80 base metal, X80 heat-affected zone (HAZ), weld metal, X60 HAZ, and X60 base metal as five distinct material regions. Miniature tensile tests provided the local constitutive input for these regions. The numerical procedure was verified against published SENT force versus crack-mouth-opening-displacement (CMOD) data; because direct SENT fracture-toughness tests for the present X80-X60 target joint are not yet available, the literature comparison is treated as verification of the modeling procedure rather than direct validation of the target joint. Parametric analyses were then performed for an initial crack ratio of a0/W = 0.10–0.30, a thickness-to-width ratio of B/W = 0.50–1.50, weld width = 10–30 mm, HAZ width = 1–9 mm, and different weld/HAZ strength combinations. The simulations show asymmetric crack-tip plastic deformation toward the lower-strength X60 side. Increasing the crack length increases crack-tip opening displacement (CTOD), whereas increasing B/W, weld width, or HAZ width generally reduces CTOD by increasing local constraint. The influence of weld strength is strongly coupled with the strength level of the adjacent HAZs. The results are therefore interpreted as numerical CTOD-response trends for a heterogeneous SENT specimen; direct experimental fracture-toughness measurements of the target X80-X60 joint remain an important subject of follow-up work.