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Stress re-distribution induced by creep relaxation around a notch loaded in tension – measurement and modeling

An abstract of a technical paper presented at:
American Society of Mechanical Engineers ASME-PVP2009 Pressure and Vessels and Piping Conference
Prague, Czech Republic
July 26–30, 2009

Prepared by:
Nicholas Christodoulou
Canadian Nuclear Safety Commission (CNSC)
Ottawa, Ontario, Canada


The majority of the pressure-retaining components in the core of a CANDU power generation system are manufactured from zirconium. The horizontal fuel channel components and the fuel bundles that contain the natural uranium fuel are manufactured using various grades of zirconium. The fuel channel consists of two concentric tubes; an internally pressurized tube (Zr-2.5%Nb) that contains the fuel bundles (Zr-4) and the re-circulating heavy-water primary coolant enclosed by a larger diameter calandria tube (Zircaloy) that separates the pressure tube from the heavy-water moderator. Refuelling and other fuel management operations can create surface defects in the tubes and fuel bundle sheathing. Stress analyses of these small notches may indicate that, under certain conditions, cracks can be formed at the root of these notches. These flaws are locations of stress concentration in the internally pressurized tube and can initiate a failure mechanism known as delayed hydride cracking. The anisotropic material properties of these zirconium components add an additional level of complexity in an analysis. However, the occurrences of these life-limiting events appear to be minimized mainly due to beneficial contributors such as stress relaxation around the scratches. One of the most likely reasons for this relaxation is thermal creep. Previously [1], the measurement and modeling of thermal creep relaxation under constant displacement was examined using 2-D finite element (FE) models. This paper extends both the measurement and modeling of the relaxing stress/strain field to the more demanding boundary condition of constant applied load. Neutron diffraction is used to determine the changing strain field around a single notched, axially orientated specimen loaded in tension. This specimen orientation and loading configuration is modeled in three dimensions using a hybrid explicit FE program [2] that contains materials subroutines that describe high stress creep specially developed to simulate the highly anisotropic creep response of pressure tube materials. Despite the difficulty of obtaining precise delineation of the moving strain field, a good agreement between the measurements and the 3-D FE creep results is achieved. Using the creep subroutines, the FE models are used to examine the creep response of a single notched, transversely orientated specimen loaded in tension in the hoop direction.

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