FRACTURE INITIATION AT HYDRIDES IN ZIRCONIUM

The effect of hydride size and stress state on fracture initiation at hydrides in a reactor grade Zr material has been studied. Uniaxial and triaxial states of stress were imposed by using smooth and notched tensile specimens, respectively. Crack initiation at hydrides was monitored using acoustic emission (AE). The specimens contained, nominally, either 0.18 or 0.90 at. pct hydrogen. Plate- or needle-shaped hydrides having different lengths were produced by varying the cooling rate to room temperature from the hydrogenation temperature. Initial orientation of the plate normals of the hydrides with respect to the tensile axis was mainly random. After deformation, the hydrides near the fracture surface were all oriented with their plate normals perpendicular to the tensile axis direction. Regardless of the hydride size, fracture at hydrides commenced at stress levels just above the proportional limit under uniaxial deformation. Average plastic strain values at initiation were approximately 0.2 pct. Slightly lower values of plastic strain were needed to initiate fracture at hydrides under triaxial loading. Fracture of the hydrides was always through-thickness and specimen fracture ductile. This is in contrast to previous results on hydride fracture obtained using the pressure tube alloys. In these materials, the fracture of hydrides with their plate normal oriented parallel to the tensile axis became less ductile as the hydride length increased.