The J-R curve behavior of irradiated nuclear pressure vessel steels is characterized in the ductile upper-shelf regime in order to provide a materials basis with which to assess the margin of safety against fracture for water reactor vessels exhibiting a low upper-shelf C_V energy. With the single specimen compliance technique, the R-curve is shown to follow a power-law behavior for small crack extension, and this phenomenon has led to a proposed new indexing procedure forJ_Ic. In addition, a specimen size dependence of the R-curve has been suggested by the results of similar compact tension specimens up to 100 mm thick and which have sufficiently deep side grooves to produce a straight crack-front extension.

R-curve data are presented in terms of a J versus T instability diagram which couples material and structural parameters, thereby permitting an analysis to be made of the margin against failure in terms of J. Also, a correlation has been suggested between the R-curve parameters and C_V shelf energy; this could enhance the structural significance of C_V reactor surveillance data.