Bounding the plastic strength of polycrystalline solids with pressurized cavities

J. E. RAMOS NERVI; M. I. IDIART

Santiago de Chile

Congreso; XIV Pan-American Congress of Applied Mechanics; 2014

Nuclear fuel pellets in CANDU reactors are made of polycrystalline natural uranium dioxide with a face-centered cubic symmetry. As a result of their fabrication process and burn up during operation, microcavities nucleate within the material at both intergranular and intragranular levels. These cavities act as sinks for the fussion gasses produced by neutron radiation. During incidental conditions, the gas pressure within the cavities can rise significantly and cause swelling of the pellet by plastic deformation. This can induce severe damage to the surrounding cladding, allowing fission gasses to diffuse to the coolant. The mechanical loads exherted by the pellet on the cladding depend on the thermomechanical behavior of the polycrystalline pellet. The elastoplastic deformation of a polycrystalline material is to a great extent dictated by the morphology, lattice orientation, and elastoplastic response of each individual single-crystal grain composing the aggregate, the porosity, and the internal gas pressure. Relating the macroscopic response with the microscopic properties is necessary to estimate the deformation-induced plastic anisotropy that develops in these materials when subjected to large deformations. Very often the response of these materials is idealized as elastically rigid and plastically non-hardening. Within this so-called rigid-perfectly plastic model, the above problem reduces to finding the macroscopic yield surface of the polycrystal given the yield surface at the single-crystal level and the statistics of the morphology and orientation distributions of the grains and cavities. Due to their inherent microstructural randomness, cognate polycrystalline solids do not exhibit a single response but a ---hopefully narrow--- range of responses. In this work we derive bounds for the range of possible responses. To that end, we make use of the of a comparison-medium method proposed by Idiart & Ponte Castañeda (Proc. R. Soc. Lond. A vol. 463, 2007). The effect of internal pressure is then introduced in the bounds via a suitable change of variables following the work of Vincent et al. (Int. J. Solids Struct. Vol. 46, 2009). The method is applied to various material systems in order to explore the simultaneous effect of crystallographic symmetry and internal pressure level on the overall plastic strength of polycrystalline solids.