Experimental investigations of the fracture toughness enhancement associated with shallow flaws

摘要

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 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 shadow-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) conditions. The HSST investigation is a joint analytical/experimental study combining the use of shallow-cracked laboratory specimens with RPV analysis. All tests have been performed on beam specimens loaded in 3-point bending using specimens about 100 mm deep. Primarily two crack depths have been considered: a = 50 and 9 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. The testing has produced a limited database of fracture-toughness values as a function of crack depth which can be used in probabilistic or deterministic fracture mechanics analyses of pressure vessel integrity. Final test results from the shallow-crack fracture toughness program will be included in this paper. Examination of previously tested thermal shock data reveals that no toughness elevation appears to be present even though the thermal shock cylinders were tested with shallow flaws.