In the nuclear power plants (NPPs), the base isolators can be used to decrease the seismic force (acceleration, shear force, floor response spectra).
During the lifetime of isolation systems, the mechanical properties of rubber bearings will be constantly changed by the vulcanization and degradation of the rubber due to environmental and chemical factors such as the thermal oxidation, ultraviolet irradiation, and ozone. Usually, thermal oxidation is the most significant degradation factors during the lifetime of natural rubber. Thermal oxidation hardens the rubber and results in a significant decrease in performance. It was presented that the stiffness was increased and damping was decreased by the aging of lead rubber bearings (LRB). It can be observed that the horizontal stiffness of rubber bearings increases about 10% after 15 to 40 years. Considering aging effect of bearings at the design stage, AASHTO [1999] suggested an aging factor for the post-yield stiffness in LRBs. The aging of LRB may be proceeded from the surface to internal because the aging of rubber bearings mainly affected by the oxidation. Therefore, the long-term behavior of LRB can different according to the size. In this study, the horizontal stiffness and damping was evaluated by the size of LRB using the analytical model.
The LRBs with an outside diameter of 250 mm (D-250) and an outside diameter of 1,500 mm (D-1500) were selected as example model. In this study, Yeo model was used to model the properties of rubber material and ABAQUS was used as the analysis program. Aging depth of LRB was assumed to be 11 mm and 16 mm based on the accelerated thermal oxidation test.
From the analysis result, the effective stiffness of LRB of D-250 was increased about 15%∼22% while the effective stiffness of LRB of D-1500 was increased about 4%∼9%. Although the aging depth of LRB was equal, the ratio aging area to total area of LRB was different according to the size of bearings. Therefore, the mechanical properties factors should be proposed considering the size of LRB.
