Cavitation in Rubber Vulcanizates Subjected to Constrained Tensile Deformation

Abstract

The deformation and failure behavior of rubbers is significantly influenced by the chemical composition and loading conditions. Investigations on how specific loading parameters affect the mechanical behavior of rubbers are elementary for designing elastomeric products. Suitable fracture mechanical concepts describing the failure behavior of rubbers are widely accepted in industrial and academic research. However, the most common failure analyses base on macroscopic approaches which do not consider microscopic damage, although a contribution of (micro)structural changes at the network scale on the overall mechanical properties is very likely. A special phenomenon in terms of microstructural failure is cavitation due to strain constraints. Under geometrical constraints, the lateral contraction is suppressed. As a result, stress triaxiality causes inhomogeneous deformation, and internal defects, so-called cavities, appear. The formation and growth of cavities release stress and reduce the degree of constraints. Cavitation in rubbers has been studied for several decades, but the knowledge about the fundamental mechanisms triggering this process is still very limited. The present study aimed to characterize and describe cavitation in rubbers comprehensively. Hence, advanced experimental techniques, such as dilatometry and microtomography, have been used for in situ investigations on pancake specimens. Such thin disk-shaped rubber samples are characterized by a high aspect ratio. As a result, the degree of stress triaxiality is high, and the dominating hydrostatic tensile stress component causes the initiation of cavitation. Of special interest was the often suspected cavitation in unfilled rubbers. In contrast to the literature, cavitation in rubbers is not exclusively attributed to interfacial failure between the soft rubber matrix and rigid filler particles, but occurs also in unfilled rubbers. The onset of cavitation was determined precisely by highly sensitive data acquisition. Both a stress-related and an energy-based cavitation criteria were found indicating that traditional approaches predicting cavitation overestimate the material resistance against cavitation. The presented experimental methods to characterize cavitation are suitable for future studies investigating further aspects of cavitation in rubbers and other rubberlike materials, e.g., the failure behavior under dynamic loading.