Experimentelle Untersuchungen und Beschreibung des deformationsinduzierten anisotropen Werkstoffverhaltens von verstärkten Elastomeren

The first uniaxial extension of reinforced rubber materials causes stress softening (the Mullins effect) not only in this loading direction but in any other direction of subsequent extension. However, the Mullins effect is less pronounced after a change of loading direction than in the direction of the initial loading. Due to the deformation history mechanical behaviour of the the rubber vulcanisate is distinctly anisotropic. To trace the anisotropic Mullins effect, firstly the standard test method for characterization of the isotropic mechanical behaviour must be extended. The appropriate type of specimen enables to perform multiple load steps with alternating load directions. After repeated stretching in the same direction, a subsequent first uniaxial loading in any other direction is characterized by a stiffer stress–strain behavior compared with the stabilized curve of the previous primary load. Due to the high relevance of the primary permanent set for this experimental method and their strong influence on the secondary load step, the direction dependent consideration is studied. On behalf of a more comprehensive evaluation of the anisotropic Mullins effect the role of active filer like carbon black and silica was investigated. In particular, not only the influence of basic properties of carbon blacks, such as specific surface area or structure, but also the influence of the amount incorporated was determined experimentally. The comparison of carbon black and silica filled vulcanisates provided more insights in the anisotropic material behaviour of reinforced elastomers. For the anisotropic (primary) Mullins effect a similar behaviour can be detected regarding the well-known one-dimensional Mullins effect. In summary, the vulcanisates soften high levelly in the primary load (one-dimensional Mullins effect) and in this way, they show a high level deformation induced anisotropy of the Mullins effect. For the visualization of the direction-dependent stress softening the comparison of the Mullins effect and the applied primary strain energy density is proposed to identify the resulting anisotropic material properties of the elastomer