A Biaxial Test Frame to Aid in Formulation of High-Fidelity Constitutive Models of Rubber Materials

Biaxial tensile testing is a complex, non-traditional but highly accurate testing technique for mechanical characterization of engineering materials. Typical materials tested in biaxial tension are metal sheets, fiber reinforced thermoplastics, thermosets, rubbers and tissues. One of the main reasons that motivates use of biaxial testing is studying the mechanical response for materials to different combinations and rates of stress-strain states, leading to development of constitutive laws that model the underlying materials more accurately. Better quality of constitutive laws provide greater insights into material performance, degradation and failure.

Worldwide many test specifications have been developed for standardized tests on rubber in uniaxial tension, compression, shear and also frequency and strain sweep based dynamic and fatigue tests. However no specific standard or protocol has been developed for biaxial testing of rubber and polymeric thin films. Biaxial tension testing has been traditionally done for metals and composite samples but there are high complexities in carrying out for rubber and thin film material samples. Due to differences in the operating range of the materials, the difference in the nature of applicable loads, and resulting stresses and strains, there have been very few test rigs for specialized biaxial testing of rubbers and thin films. Biaxial test rigs whether in the commercial or academic domain are often customized to serve for specific target materials and their test conditions. Apart from biaxial test rigs, fixtures that provide biaxial deformation and designed to be used on a single axis universal testing machines have also been developed and used.  

Four (4) types of tests can be carried out in the test rig;

  1. Single Stretch.
  2. Multiple Cyclic Loading.
  3. Single Stretch followed by Stress Relaxation Step.
  4. Multiple Cyclic with Stress Relaxation Steps.
Figure 1: AdvanSES Biaxial Loading Test Frame