The Heavy Section Steel Technology Program (HSST) is investigating the influence of flaw depth on the fracture toughness of reactor pressure vessel (RPV) steel. Recently, it has been shown that shallow cracks tend to exhibit an elevated toughness as a result of a loss of constraint at the crack tip. The HSST investigation is a joint analytical/experimental study combining the use of shallow-cracked laboratory specimens with finite element analysis. All tests have been performed on beam specimens loaded in three-point bending using specimens about 100 mm deep. Primarily two crack depths have been considered: a = 50 and 10 mm (a/W = 0.5 and 0.1). Test results indicate a significant increase in the fracture toughness associated with the shallow-flaw specimens in the lower transition region compared to the conventional fracture toughness. Little or no toughness increase is present on the lower shelf where linear elastic behavior takes place. In addition, the test data indicate that specimen thickness had little influence on fracture toughness for either the deep- or shallow-crack specimens.
Posttest analyses were performed for the deep- and shallow-crack specimens to compare the experimental and analytical toughness values and to quantify the constraint in terms of the Q-stress parameter. The Q stress at initiation was found to be about 0 for the deep-crack beam and about -0.7 for the shallow-crack beams. These results indicate no loss of constraint in the deep-crack specimens but a significant constraint loss in the shallow-crack specimens, which is consistent with observed behavior. The posttest analysis has been used to construct a Jc(Q) locus of toughness data.