Unified Mechanics Theory - Finite Element Modeling on Mechanical Damage for Lead-Rubber Bearings

Abstract

Several studies have been developed to estimate the reduction of the characteristic strength of the Lead Rubber Bearing (LRB) respect to the increment of the cycles. This phenomenon has been observed in several experimental studies involving cyclic motion, but no means to predict this behavior using analytical analysis, and involving temperature changes of the lead has been reached yet. LRBs are used worldwide as devices for seismic isolation of buildings and bridges, for new structures and retrofit projects. This document presents the first attempt for the implementation of the Unified Mechanics Theory to simulate entropy generation (or damage evolution) of the mechanical properties of high purity lead core of a Base isolator type LRB. A finite element model was made in ABAQUS using 3D elements. Thermodynamic based constitutive models comprise the UMAT subroutine to update the Jacobian matrix of the material properties of the lead core, for each time increment with the respective temperature increment associated. These constitutive models involve, viscoplasticity defined by a yield surface, kinematic hardening and finally damage in the material. The results are compared to experimental test done at State University of New York at Buffalo (UB) by (Kalpakidis & Constatinou, 2008), specifically the example No 10 was taken as the validation test, which dimensions are described as “typical” for an LRB in base isolation in Latin America. After several computationally expensive versions of the model, convergence was reached and degradation of the lead core was observed, the shear force obtained was bigger than expected. However, the energy dissipated by cycle has similar tendency to the actual laboratory test, with adjusted values.