Ductile Fracture Initiation in Compact Tension, Pressure Tube and Curved Compact Tension Specimens of Hydrided Irradiated Zr-2.5Nb Materials with Split Circumferential Hydrides

Abstract

Zr-2.5Nb pressure tubes are primary containment components in CANDU (CANada Deuterium Uranium) reactors. One of the important reactor safety concerns is the fracture of the Zr-2.5Nb pressure tubes due to precipitation of circumferential hydrides. In this investigation, ductile fracture initiation with consideration of strain concentration and stress triaxiality near crack fronts in compact tension (CT), pressure tube (PT) and curved compact tension (CCT) specimens of hydrided irradiated Zr-2.5Nb pressure tube materials with split circumferential hydrides is investigated by three-dimensional finite element analyses with submodeling. The stress-strain relation for the irradiated Zr-2.5Nb pressure tube materials is taken as elastic plastic strain hardening with the perfectly plastic behavior at large plastic strains based on the experimental results of transverse tensile tests. A strain-based failure criterion with consideration of stress triaxiality is developed from the Gurson yield model with use of the experimental/computational results of transverse tensile tests and the effective plastic strain of the critical material element in CT specimens without hydride at fracture initiation. The results of the three-dimensional finite element analyses of CT, PT and CCT specimens of irradiated Zr-2.5Nb pressure tube materials without circumferential hydrides suggest that circumferential hydrides ahead of the crack front in the middle of CT, PT and CCT specimens should fracture with the assumption of the hydride fracture stress of 750 MPa for the given load or internal pressure that corresponds to the same fracture toughness (Kc) of the irradiated Zr-2.5Nb pressure tube materials without circumferential hydrides. Based on a strain-based failure criterion with consideration of stress triaxiality, the fracture initiation is determined, respectively, in the middle of the thickness for CT specimens, near the middle of the thickness for PT specimens, and at a quarter of the specimen thickness from the inner specimen surface for CCT specimens. The fracture initiation load or internal pressure corresponding to the same Kc based on the strain-based failure criterion with consideration of stress triaxiality for CCT specimens without hydrides is 2% lower than that for CT specimens without hydrides and is 4.3% lower than that for PT specimens without hydrides. The strain-based failure criterion with consideration of stress triaxiality can be used to describe the slightly higher fracture toughness for a PT specimen due to different constraint conditions and different types of loading, compared with that for a CCT specimen at room temperature. For CT, PT and CCT specimens with split circumferential hydrides with various heights and ligament thicknesses, plastic strain concentration is observed in the middle of the ligament ahead of the crack front when the ratio of the ligament thickness to the hydride height is less than 3. With the strain-based failure criterion with consideration of stress triaxiality, the necessary fraction of the load for crack initiation is about 0.60 to 0.70 to fracture the ligaments when compared to that for CT, PT and CCT specimens without split circumferential hydrides. The computational results can be used to explain the near 35% reduction of the fracture toughness at room temperature obtained from hydrided irradiated PT and CCT specimens when compared with those from unhydrided irradiated ones.