Response of Carbon Black Filled Butyl Rubber to Cyclic Loading

Abstract

Abstract The stress softening behavior of a carbon black filled butyl rubber in cyclic loading (creep) and cyclic deformation (stress relaxation) histories was investigated. Single step and multiple step stress relaxation and creep experiments were conducted as were creep experiments with sinusoidal loading histories. The data were analyzed in view of the BKZ constitutive equation. For materials which follow the behavior predicted by the BKZ theory, the single step stress relaxation response is a lower bound for the stress in cyclic deformation histories, and the single step creep response is an upper bound for the deformations obtained in cyclic loading histories. In our experiments we observed that for filled butyl rubber, the responses to cyclic deformation and cyclic loading histories were outside these bounds. The softening was much more emphatic in cyclic loading histories than in cyclic deformation histories. We also found that the rubber is stiffer in creep than predicted from the BKZ theory (from stress relaxation data). It is suggested that an adequate description of material behavior might be obtained using a theory similar to the BKZ theory, which includes competing effects of stiffening in creep and softening under load-unload cycling. The failure lifetime of the filled butyl rubber in cyclic loading histories was related to the stress softening behavior. If it is assumed that, in some sense, a failure envelope exists, then it can be shown that under cyclic loading the filled butyl rubber should fail at a time earlier than under constant load conditions. We were able to predict failure lifetimes by using creep data obtained from short time cyclic loading histories and extrapolating to the strain at break determined from a shifted failure envelope. This approach relates the observed frequency and waveform dependence of failure lifetimes to the creep behavior under cyclic loading conditions.