Impact of the Direction Dependent Stress Softening On the Multiaxial Material Behavior of Filled Rubber

The mechanical behavior of filled rubbers depends on the maximum stretch load and on the induced stress softening. This softening effect is referred to as the Mullins effect. Current investigations point out that the deformation induced Mullins effect is a very complicated issue. In particular, the significant directional dependence of the Mullins effect asks for anisotropic material models. But for the formulation and validation of anisotropic material models there is still a lack of suitable experimental data.

In order to trace the anisotropic Mullins effect the standard test method for characterization of the isotropic mechanical behavior must be extended. The appropriate type of specimen enables the performing of 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 to the stabilized curve of the previous primary load. So the experimental results confirm the deformation induced anisotropy.

In order to identify the multiaxial material behavior after the pre-stretching in one direction, a biaxial tensile testing machine is developed. The specific property of the biaxial tensile testing machine is the independence of the both loading axes. So the rubber material can be applied with arbitrary loading histories. Therefore a cross shaped specimen with four arms is used. A special form of multiple slitting parallel to the sides on each arm ensures the homogenous uniaxial load-condition in the primary load. In the secondary load step the loading axis, which has previously been inactive, is moved translational in the uniform manner as the master axis or in a defined ratio. The experimental results confirm the deformation induced anisotropy of the Mullins effect.