Experimental characterization and modelling of the cyclic softening of carbon-black filled rubbers

Several carbon-black filled styrene-butadiene rubbers (SBRs) were submitted to uniaxial tension cyclic tests at large strain for investigating filled rubbers cyclic softening. In order to study the effect of the material composition (amount of fillers and crosslink density) and of the stretch intensity, an original method for cyclic softening characterization is proposed. The softening is seen as an amplification of the stretch in the rubber gum, and a stretch amplification factor is then introduced as the softening parameter. The latter is evaluated for various materials and several levels of cyclic stretch intensity. Material softening is shown to increase with the loading intensity, the amount of carbon-black fillers and the decrease of the crosslink density. In terms of modelling, the introduced softening parameter provides access to the entire stress-stretch responses of the softened material for any cycle. Therefore, a model is written for the description and the prediction of the stress-stretch responses of filled rubbers during proportional cyclic tests.