Experimental Investigation of Crack Tip Constraint Effects on Fracture Assessment of API 5L X65 Steel Grade for Low-Temperature Applications (−120°C)

Crack tip constraint is a significant issue in engineering components’ design and repair decisions. The main reason is that fracture assessment procedures, such as BS 7910, rely on lower-bound fracture toughness test data from deeply cracked bend specimens. This can generate stress states under various loading conditions with an appropriate crack tip stress triaxiality for metallic structures. Many real components (e.g., oil and gas pipelines) have small in-plane (shallow cracks) and out-of-plane (thin-wall thickness) dimensions that can cause a reduction in crack tip constraint to a considerable amount, thereby increasing the fracture toughness. As such, the structural assessment of low-constraint structural components using fracture toughness data obtained from deeply notched specimens may be safe but overly conservative, resulting in unnecessary repair shutdowns and costs. Consequently, relating fracture toughness values determined from laboratory specimens to real structural components becomes an issue in structural integrity assessments based on the two-parameter fracture mechanics methodology. This study investigates the applicability of the constraint-based failure assessment diagram (FAD) approach for the evaluation of cracked pin-loaded single-edge notched tension and three-point single-edge notched bend specimens at low (−120°C) and room temperatures. The analyses reveal that the experimentally measured toughness values, J₀, depend on the crack sizes for the considered specimen geometries (a/W = 0.1, 0.3, 0.5). The results show the benefits of using constraint-modified FAD approach for the assessment of shallow cracks. Therefore, the enhanced toughness associated with constraint reduction indicated an increased margin and allows realistic design and repair decision-making that can help prevent catastrophic failures.