The Earthquake Engineering Online Archive

Material characterization of elastomers used in earthquake base isolation

Papoulia, Katerina D.; Kelly, James M.

UCB/EERC-90/18, Earthquake Engineering Research Center, University of California, Berkeley, 1990-12, 71 pages (515/P228/1994)

The material properties of filled rubber are investigated and a three-dimensional finite strain constitutive model of rubber viscoelasticity is developed within the context of continuum mechanics with internal variables. A fractional derivative model, which agrees well with one-dimensional experimental data, is employed to describe the short-time and cyclic behavior in the frequency range typical of most earthquakes. The model is extended to the nonlinear case through the use of a free-energy function, consistent with the assumption of an additive stress decomposition motivated by the simple Kelvin model of linear viscoelasticity. A constitutive law for the inelastic part of the stress is provided. A formulation for deviatoric and volumetric response is considered, and it is assumed that the volumetric response of the material is elastic. The elasticity of rubber is modeled following classical models extended to include compressibility. Because of the highly confined condition of rubber in isolation bearings, the effect of compressibility is important. Step-by-step integration of the constitutive law is performed, and the effect of the number of history terms retained in the analysis is examined in connection with the fading memory property of the material. Simple shear experiments are used to assess the predictive capabilities of the model.

Available online: http://nisee.berkeley.edu/documents/EERC/EERC-90-18.pdf (2 MB)