The extent to which credit can be taken for the increased toughness associated with shallow cracks in PTS probabilistic fracture mechanics analyses

Abstract

Shallow flaws play a dominant role in the probabilistic fracture mechanics analysis of a reactor pressure vessel during a pressurized thermal shock (PTS) event. Recent research has shown that the initiation toughness associated with a shallow flaw is greater than the conventional toughness associated with a deep crack, and this enhanced toughness has been shown to have a marked favourable impact upon the conditional probability of vessel failure. The analyses that have led to this conclusion have been based upon an initiation criterion where the linear elastic stress intensity factor is equated with the appropriate initiation toughness value. This paper shows that this criterion is non-conservative when the initiation temperature exceeds RT NDT for shallow cracks up to 10 mm deep, and when the initiation temperature is less than RT NDT for very shallow ( ∼5 mm deep) cracks. Consequently the anticipated favourable impact of taking credit for the enhanced shallow crack toughness might not be as great as has been envisaged.