A Review of Fracture Toughness Testing and Evaluation Using SENT Specimens

Abstract

Fracture toughness is an important material property in describing material resistance against fracture with a point value or in the format of a resistance curve. For ductile materials, the commonly used fracture parameters are the J-integral and the crack-tip opening displacement (CTOD, or δ). ASTM E1820 provides standard procedures for determining the JIc, δIc, J-R curve and δ-R curve using bending specimens with deep cracks. This usually leads to high crack-tip constraint conditions and conservative fracture resistance curves. Actual cracks found in pipelines and welds are often shallow and dominated by tensile forces, resulting in low constraint conditions and elevated resistance curves. Thus the standard resistance curves can be overly conservative for a shallow crack. To obtain realistic fracture toughness values to meet the practical needs for pipelines, different test methods have been developed using a single edge-notched tension (SENT) specimen. This includes the multiple specimen method, the single specimen method, the J-R curve test procedure, and the δ-R curve test procedure. This paper presents a critical technical review of existing fracture toughness test methods and procedures using SENT specimens, with discussions on the toughness estimation equation, key parameter calibration, rotation correction, and test procedure limitation. Historical efforts related to the SENT testing and applications of ASTM fracture test standards to the SENT specimens are also reviewed briefly.