How to effectively perform equibiaxial tension for rubber materials?

The utilization of a reasonable rubber hyperelastic constitutive model (HCM) is fundamental to obtain high-precision simulation results for the reliability analysis and structural optimization of rubber products under complex working conditions. Equibiaxial tension (ET) is of importance for the material parameters identification of HCMs. To date, there is no ISO standard for ET test of rubber, nor is there a comprehensive account of the design of different ET testing methods. In this work, we focus on the fundamental challenges of ET, including (i) how to select the appropriate shape and size of rubber samples (square, cruciform-Ⅰ, cruciform-II and circular)? (ii) How to determine the reasonable strain gauge zone through optical extensometers or digital image correlation technique? (iii) How to determine the load-bearing area in the solution of nominal stress? The rationality of the uniform equibiaxial strain calibration area and the precise stress calibration area of square and circular shape samples were verified through experimental data of different rubber materials. This work could provide a theoretical support for the selection and optimization of ET testing methods for rubber materials.