A relatively simple nonlinear viscoelastic constitutive model for particle-filled rubber under three-dimensional stress states is developed from an existing axisymmetric constitutive equation and then experimentally verified. In extending the existing model to three dimensions, it is assumed that the damage leads to transverse isotropy with the axis of isotropy coinciding with the local, instantaneous maximum principal stress direction for monotonic loading. Rate-type evolution laws are used to account for the time-dependent changes in damage, and viscoelasticity of the rubber matrix is explicitly taken into account by using so-called pseudo variables; although strains may be large, in this theory rotations must be small. The model has been implemented in a finite element analysis to account for various geometry and loading conditions. Experiments were used to check the model. Various biaxial specimens of different aspect ratios, with and without holes or cracks, were tested with different cross-head rates. Load-deformation information reveal good agreement between theory and experiments, which is far better than using linear theory. A computer code based on the digital image correlation method has been developed and refined for accurate experimental displacement data extraction. The displacement field was further processed with a smoothing program fbr noise reduction. The specimen surface strain distribution is compared to predictions from theory. Nonlinear theory shows better agreement with experimental strain fields than linear theory. (C) 1998 Elsevier Science Ltd. All rights reserved.
