Review of fracture toughness test methods for ductile materials in low-constraint conditions

Actual cracks in pressure vessels and welds are often shallow ones and dominated by tensile forces, leading to low crack-tip constraint conditions and rising resistance curves. As a result, the standard resistance curves could be overly conservative for a shallow crack in real structures. To determine more realistic 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 review on 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 using high and low-constraint specimens for developing a family of constraint-dependent resistance curves, and (3) direct test methods for low-constraint specimens of single edge-notched tension (SENT) or shallow-cracked bending specimens. Basic concepts, test methods, estimation equations, test procedures, limitations, historical effects and recent progresses are reviewed.