Volume Variation in Stretched Natural Rubber: Competition between Cavitation and Stress-Induced Crystallization

The present paper deals with the competition between cavitation and stress-induced crystallization during the deformation of cis-1,4 polyisoprene rubber. During deformation, this kind of material exhibits volume variation induced by both phenomena. In this study, we propose to measure this volume variation by an original full-field measurement technique. The high resolution of this technique allows us to identify characteristic stretch ratios during mechanical cycles. More especially, the competition between cavitation and stress-induced crystallization is discussed related to stretch ratios at the beginning of crystallization and at the end of crystallite melting. Moreover, the addition of fillers significantly influences the volume variation response. They amplify the cavitation phenomenon and allow the crystallization to begin at a lower stretch ratio. Results obtained with filled compound under cyclic loading conditions show that relative volume change is stabilized for a lower number of cycles than the stress-stretch response and that the crystallinity seems to be lower than for unfilled natural rubber. Finally, for the applied strain rate, the Mullins effect is found to have no influence on the value of stretch ratios at the beginning of crystallization and at the end of crystallite melting.