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

Abstract

Fracture toughness is an important material property to describe the ability of a material containing a crack to resist fracture, and often characterized by the fracture parameters of J-integral or crack-tip opening displacement (CTOD) for ductile cracks. ASTM, BSI and ISO have developed their own standard test methods for measurement of initiation toughness and resistance curves in terms of J and CTOD using bending dominant specimens. Such bending specimens are typically designed with deep cracks to maintain high constraint conditions at the crack tip so as to determine conservative fracture toughness or resistance curve for the test material.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.