The Heavy Section Steel Technology Program (HSST) at Oak Ridge National Laboratory (ORNL) is investigating the influence of flaw depth on the fracture toughness of reactor pressure vessel (RPV) steel. Recently, it has been shown that, in notched beam testing, shallow cracks tend to exhibit an elevated toughness as a result of a loss of constraint at the crack tip. The loss of constraint takes place when interaction occurs between the elastic-plastic crack-tip stress field and the specimen surface nearest the crack tip. An increased shallow-crack fracture toughness is of interest to the nuclear industry because probabilistic fracture-mechanics evaluations show that shallow flaws play a dominant role in the probability of vessel failure during postulated pressurized-thermal-shock (PTS) events.

Tests have been performed on beam specimens loaded in three-point bending using unirradiated RPV material (A533 B). Testing has been conducted using specimens with a constant beam depth (W = 94 mm) and within the lower-transition region of the toughness curve for A533 B. Primarily two crack depths have been considered: a = 50 and 9 mm (a/W = 0.5 and 0.1). Three specimen thicknesses (B = 50, 100, and 150 mm) have been used to examine the influence of different out-of-plane constraint conditions on the test results. All tests resulted in cleavage failures. Test results indicate a significantly higher fracture toughness associated with the shallow flaw specimens compared to the fracture toughness determined using deep-crack (a/W = 0.5) specimens. The toughness increase is comparable with the toughness increase found at the University of Kansas using steels whose stress-strain properties bound those of A533 B. Test data also show little influence of thickness on the fracture toughness for the current test temperature (-60°C). The Irwin β_c correction has been modified to account for shallow flaws and was used to estimate the shallow-flaw toughness based on the results from the deep-crack specimens.