VALIDITY OF TOUGHNESS MEASUREMENTS FROM MINIATURE SPECIMENS FAILING IN DIFFERENT FRACTURE MODES

Using 0.16T (4 mm thick) compact tension 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. Achieving validity in the toughness measurements requires more care in specimen manufacture, load train alignment and test temperature selection than when larger specimens are used. This is particularly evident in the shapes of precrack fronts, which appear more sharply asymmetric on smaller specimens. Questions have also been raised as to whether the failure mechanisms are the same in miniature and large specimens. 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. The fractography shows that, even in a ductile weld metal exhibiting clear shear lip, 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 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 0.16T and larger specimens. These fractographic observations support the thesis that toughness measurements made on miniature compact tension specimens reflect the same material characteristics and failure mechanisms as those on larger specimens. The limitations on specimen geometry and loading identified by the ASTM Standard are sufficient to ensure that transition temperature values derived from miniature compact specimens are valid for materials failing via a variety of fracture modes.