Contribution to the Evaluation of the Circumferentially-Cracked Round Bar for Fracture Toughness Determination of Reactor Pressure Vessel Steels

Marc Scibetta

Public defence of the doctoral Thesis at the University of Liège,
Site du Val Benoît, Institut de Mécanique, 21, Rue Ernest Solvay, B-4000 Liège
on Friday 18 June 1999 at 16h.

The measurement of fracture toughness is based on standard specimens such as Compact Tension (CT) or Single Edge Notched Bend (SENB) specimen. However, specimen size requirements to obtain valid measurements generally lead to large or very large specimens. In this context, there is an increasing interest for the use of the small Circumferentially-Cracked Round Bar (CRB). The presumed weaker size requirement is not the only advantage of the CRB. The low cost to machine the specimen, the rotating bending fatigue allowing for easy precracking of specimens, the use of a standard tensile test fixture and the axisymmetry of the specimen that avoids time consuming 3D finite element calculations, are one by one very attractive features.

However, fracture toughness measurements using CRB's are unreliable as long as no adequate, validated and recognised testing method is used. In this work, different aspects of the CRB in fracture mechanics are investigated, such as: the precracking, the fracture toughness formulation, the size requirements for CRB specimens, the validation in the transition region including the effect of irradiation and the application of the CRB at the upper shelf temperature region.

The theoretical and experimental work demonstrate the capabilities of miniature CRB's in the transition region. It also points out disadvantages such as loss of constraint for small CRB and the early ductile instability, that make the geometry unsuitable for J-R curve determination.