Towards biaxial fatigue experiments of elastomers using square and cruciform geometries in planar tension conditions

Soft solids such as natural rubber, hydrogels, and biological tissues have anisotropic properties and are subject to cyclic biaxial loading during their service lives. This requires biaxial loading rather than uniaxial; however, no agreed standard protocol exists. Therefore, the aim of this study is to provide preliminary suggestions for reliable and consistent biaxial fatigue tests so that a good degree of biaxiality and efficiency can be thoroughly achieved. Several biaxial loading tests (equi-biaxial and unequal-biaxial) were conducted on natural rubber and ecoflex using different geometries including cruciform and square configurations. Three criteria were defined to evaluate the equi-biaxiality performance: (i) test function related to homogeneous strain distribution in the field of interest, (ii) degree of efficiency corresponding to the ratio of strain in the field of interest (gauge section located in the middle regions) to the maximum strain far from the middle area, and (iii) strain ratio, used for fatigue test. Results showed that cruciform geometry underperformed in equi-biaxiality criteria, i.e., samples possessed high uniaxial strain in the arm whereas the simple square geometry could reach a higher degree of biaxiality and efficiency. The highest equi-biaxiality performance was obtained for the optimized square geometry in such a way that a maximum equi-biaxial strain of 65 % was achieved in the field of interest while possessing a degree of efficiency of 0.66 and strain ratio of 1.96. A successful unequal-biaxial fatigue test of up to two million cycles was conducted on the optimized square specimen made of ecoflex. Finally, a new square configuration with circular cavity in the middle was suggested for future biaxial characterization and standardization of biaxial tests in which the numerical study yielded a degree of efficiency of 1 and strain ration of 2.12 manifesting a considerable improvement in the biaxiality performance. • Providing preliminary suggestions for reliable and consistent biaxial fatigue tests for soft polymers • Several biaxial loading tests (equi-biaxial and unequal-biaxial) were conducted on natural rubber and Ecoflex using cruciform and square configurations • Three criteria were defined for equi-biaxial characterization including test function, degree of efficiency, and strain ratio • Cruciform geometry underperformed in equi-biaxiality criteria with respect to the modified square configurations. • A new square configuration with circular cavity in the middle was suggested for future biaxial characterization