Zircaloy is commonly used for the cladding or pressure tubes in commercial nuclear reactors because of its strength, corrosion resistance, and low absorption of thermal neutrons. Fracture toughness test techniques using small samples fabricated from archival materials from the N Reactor pressure tubes of Zircaloy-2 were developed to study the factors affecting tube fracture toughness. Compact tension specimen thickness was limited by the wall thickness (7 mm) of the tubes. Specimens (5 mm thick) were prepared for fracture toughness testing, and results were analyzed using the J-integral approach. To reduce the high cost of irradiated specimen testing and to more easily precrack specimens remotely, single-specimen potential drop techniques were employed to evaluate the fracture toughness of Zircaloy-2.

This paper addresses problems involved in measuring fracture toughness of thin pressure tube material which, in the presence of radial-axial hydrides, undergoes a significant brittle to ductile fracture transition. Compact tension specimens (~5 mm thickness) are machined from flattened tensile strips of Zircaloy-2 in which radial hydrides (30 to 100 ppm hydrogen) are produced by precipitation under stress. Axial fracture toughness is determined for the unirradiated material between room temperature and 300°C using the dc potential drop method. At low and intermediate temperatures crack growth is governed predominantly by the presence of the radial hydrides, and the potential drop is shown to underestimate crack extension due to short-circuiting across tight crack faces. In the upper shelf regime where crack extension is governed mainly by the flow properties of the matrix, the potential drop overestimates crack extension due to through-thickness yielding. It is shown that good, reproducible results can be obtained by careful data analysis using individual specimen calibrations.