Size, geometry, and material effects in fracture toughness testing of irradiated Zr-2.5Nb pressure tube material

The effect of initial crack size on the crack growth resistance (J-R) curves determined from burst tests on irradiated Zr-2.5Nb pressure tubes has been studied and the results compared with those obtained from matched curved compact specimens. The study used sections from three different tubes representative of material of low, intermediate. and high toughness. In each case a series of burst tests was conducted at 250 degrees C with different starting crack sizes (from 35 to 85 mm) followed by small specimen testing. The toughness was characterized by means of deformation J-R curves using the d-c potential drop method to measure stable crack growth. Fractographic studies were also conducted in support of the J-R curve results. For tubes of low to intermediate toughness there is little evidence of a crack size effect on the J-R curves from the burst tests. However, for burst tests on tubes of higher toughness there is an increase in out-of-plane bending associated with the wider crack openings (bulging) that can promote earlier failure by slant/shear instability and a lowering of the J-R curve. Thus the J-R curves from such tubes exhibit more variability in toughness as well as an apparent crack size dependence. Such crack growth behavior overrides any increase in material toughness with irradiation temperature as revealed by the small specimens.