Numerical Investigation of Crack-Tip Constraint Parameters in Straight Pipes Under Reactor Operating Conditions

Abstract

J-R curve is established as fracture toughness for designing critical nuclear components. The original idea was that a single parameter, J-integral can be used to characterize the crack tip conditions and one unique fracture toughness curve is sufficient to characterize the material. However, later on it is realized that J-R curve is geometry dependent for considerable plasticzone size ahead of crack tip. The influence of crack tip stress triaxiality has been already emphasized in geometry dependent fracture resistance. Hence, two parameter fracture mechanics has emerged like J-q, J-Q, J-A2, etc. Due to simplification and applicability over wide range of loading J-q has been studied extensively for fracture specimens. Very few attempts have been made for numerical investigation of q in real life components. In Indian PHWR, PHT piping components are used to transfer hot (300oC) heavy water under internal pressure to harness the heat from reactor. The material used in piping component is SA333Gr6, Carbon Steel (CS) material. In the present work, three dimensional finite element models are prepared for Three Point Bend Bar (TPBB) specimen and straight pipes with different crack configurations like throughwall and surface cracks. Pipes are simulated under loading of four point bending. The load is applied monotonically and quasi-statically to study the fracture behaviour. These pipes are analyzed under internal pressure or elevated temperature. Four point bending is performed with both of internal pressure and elevated temperature also which is actual reactor operating conditions for pipes. q and Q parameter are calculated for the TPBB specimen and pipes. The work also addresses the variation of q and Q for throughwall crack and surface crack in remaining ligament with deformation level. This data is useful for transferring the laboratory specimen J-R curve to the component level considering stress triaxiality.