Validity of Toughness Measurements From Miniature Specimens Failing in Different Fracture Modes

Abstract

Using mini-C(T) (4 mm thick, 0.16T) specimens to determine toughness in reactor pressure vessel steels permits the ductile to brittle transition temperature to be derived from small amounts of material and allows more effective use of surveillance specimens. However, questions have been raised as to whether the failure mechanisms are the same in miniature and large specimens, something that must be ensured when transferring fracture results obtained in mini-C(T) specimens to larger components. This work, performed within the FRACTESUS project, presents toughness measurements and detailed fractography on both a homogeneously brittle base metal and a relatively ductile, inhomogeneous weld to assess the transferability of fracture data. The fractography shows that brittle fracture initiates within the part of specimen experiencing small scale yielding, so long as the toughness measurement is valid. Similarly, although the precrack front asymmetry appears more marked in smaller specimens, as long as the deviation from planarity is within the ASTM E1921 limits, the asymmetry does not affect the location of the initiation site. For materials showing a variety of fracture modes, the fracture modes observed at the initiation sites are consistent with those observed in larger specimens. Where data are available, the stress and strain conditions at the initiation sites are also found to be consistent in mini-C(T) and larger specimens. These observations support the thesis that toughness measurements made on mini-C(T) specimens reflect the same material characteristics and failure mechanisms as those made on larger specimens.