Advances in Fracture Toughness Test Methods for Ductile Materials in Low-Constraint Conditions

Fracture toughness is an important material property in the fracture mechanics methods, and often described with the fracture parameters of J-integral and crack-tip opening displacement (CTOD) for ductile cracks. ASTM, BSI and ISO have developed their own standard test methods for measuring initial toughness and resistance curves in terms of J and CTOD using deeply cracked bending specimens. Such specimens are of high crack-tip constraints and give conservative toughness or resistance curves.Actual cracks in pressure vessels and welds are often shallow and dominated by tensile forces, resulting in low crack-tip constraint conditions and rising resistance curves. Thus the standard resistance curves could be overly conservative for a shallow crack in real structures. To obtain more reasonable fracture toughness for ductile cracks in low-constraint conditions, many experimental and analytical methods have been developed in the recent decades. This paper presents a critical technical review of fracture toughness test methods for standard and non-standard specimens, including (1) ASTM, BSI and ISO standard test methods for high-constraint specimens, (2) constraint correction methods for determining a family of constraint-dependent resistance curves, and (3) direct test methods for a low-constraint specimen: single edge-notched tension (SENT) specimen. This includes the basic concepts, basic methods, estimation equations, test procedures, limitations, historical effects, and recent progresses.