In situ dilatometry and X-ray microtomography study on the formation and growth of cavities in unfilled styrene-butadiene-rubber vulcanizates subjected to constrained tensile deformation

It is known, that under constrained tensile deformation the mechanical response and the failure behavior of rubber vulcanizates reveal unlikely features. Due to multiaxial stress concentrations, the formation of internal cracks can be initiated. For the first time, fundamental investigations on unfilled styrene-butadiene-rubber pancake specimens were performed. Via in situ dilatometry and X-ray microtomography experiments, the damage process under constrained tensile deformation is studied and discussed. Although cavitation has been often discussed for filler reinforced rubber vulcanizates, the results of this study demonstrate how it occurs also in unfilled rubbers. In fact, the stronger the geometrical constraints are, the higher is the number of small cavities. However, the integral cavity volume is not affected. Moreover, micrographs of fracture surfaces indicate that cavitation is controlled by an omnidirectional growth of radial side-cracks. Finally, an energy-based approach to describe the cavitation onset criterion initiating the damaging process is presented to revise Gent's theory.