3.1 This test method provides a measure of rheological non-linearity for filled rubber compounds under oscillatory shear conditions: the normalized 3rd harmonic of the torque I_3/1.

3.2 Rheological linearity means that the modulus is a function of frequency only. The shear modulus dependency on both frequency and amplitude of a dynamic deformation is a non-linear, rheological effect. Filled rubbers show a strain dependency of the modulus known as Payne effect. A test method for evaluating the Payne effect can be found in Test Method D8059.

3.3 One of the main contributions to the Payne effect is the so-called polymer-filler interaction in the range of mid amplitude oscillation shear, MAOS. The MAOS amplitude range is defined as the range where I_3/1 is already measurable and increases according to a scaling law, for example, I_3/1 ~ ?². It has been shown that FT rheological measurements are very sensitive to changes in this interaction, and that it is possible to quantify the influence of the filler type and content on the nonlinearity. Interactions between the polymer and particle surface and between the filler particles create a network structure in the compound that not only increases the elasticity of the system but also introduces nonlinear contributions to the stress response.3