Cavitation damage in seismic bearings of existing nuclear facilities

Rubber isolation systems represent an effective method to isolate the superstructure from seismic ground accelerations. In this study, a high-damping natural rubber bearing is subjected to characterization tests to determine corresponding stiffness parameters. The geometric layout of the bearing shows a number of inclined steel plates to obtain a transversely non-isotropic response. It is shown that during shear deformation a large negative hydrostatic stress state and irreversible stress softening attributed to cavitation damage develops in localized areas. The cavitation region is not visible from the outside and therefore not recognized or accounted for during experimentation. For a hydrostatic tension above a critical value an elastic instability develops with a sudden reduction in the value of the tensile bulk modulus of the material during loading and significant stress softening on unloading. As a result of the cavitation damage the tensile bulk modulus in the natural configuration is permanently reduced. Ultimately, failure of the material occurs at sufficiently large hydrostatic tension, typically when the negative pressure locally exceeds a critical value. Numerical simulation using a cavitation damage model based on a variable bulk modulus quantifies the amount of stiffness reduction attributed to internal rupture in the material. (author)