Experimental and numerical evaluation of stress relaxation of engine rubber

Compressive stress relaxation tests of rubbers were performed at 10% to 50% peak strains. The hyperelastic and viscoelastic parameters were experimentally evaluated and used for the finite element analysis (FEA) to predict the compressive stress responses under various applied strains and geometries. The numerical predictions were in good agreement with the experiments. During hyperelastic period, the slipping of initial contact edge was possible at low peak strains (10% to 30%), while the slipping of initial contact and the formation of new contact occurred at intermediate peak strains (30% to 40%). At high peak strains (40 to 50%), the combination of slipping of initial contact, formation of new contact and slipping of new contact was possible. During viscoelastic period, the relaxation of vertical stress involved the release in strain energy from the movement of molecular structures, thus it mainly occurred in the bulk of rubber. On the other hand, the relaxation of lateral stress involved the decrease in friction force at contact, thus it mainly occurred around the contact. These findings provided some guidelines for engineers to numerically evaluate the influence of applied strain, geometry, and friction at contact on the stress relaxation behavior of rubber.