Ultimate tensile properties of elastomers. V. Rupture in constrained biaxial tensions

The strength and extensibility of an unfilled styrene‐butadiene rubber vulcanizate were determined in constrained biaxial tension (essentially pure shear) by stretching thinwalled cylindrical specimens in the axial direction with an Instron tester while gas pressure was admitted to maintain constant the outside diameter of a specimen. The rupture stress, λ 1b , and the extension ratio, λ 1b , in the axial direction and the rupture stress, σ 2b , in the circumferential direction were determined at extension rates from 0.0031 to 3.1 min −1 and temperatures from 25 to 90°C. Rupture data in simple tension were also obtained by testing ring specimens under similar test conditions. The time‐temperature shift factor, a T , obtained by superposing rupture data at different temperatures, gives a constant activation energy of 35 kcal. Doubly logarithmic plots were prepared showing the dependence of σ 1b /λ 1b , σ 2b , and λ 1b from the biaxial tensile tests and of σ b /λ b and λ b from the simple tensile tests on the temperature‐reduced extension rate, {article}{empty}{document}λ aT{document} . (The quantities σ 1b /λ 1b , σ 2b , and σ b /λ b are rupture stresses based on the dimensions of undeformed specimens.) At equal values of {article}{empty}{document}λ aT{document} , the extension ratios λ 1b and λ b are sensibly identical. The ratios σ 1b /σ b and σ 2b /σ b , as well as λ b σ 2b /σ b , were obtained from rupture data read from the composite curves at selected values of {article}{empty}{document}λ aT{document} . Also, similar ratios were derived from stress data at equal extensions in biaxial and simple tension for 1.25 ≤ λ ≤ 3.0, the range comparable to that in which rupture data were obtained. A consideration of the ratios given by both the rupture and the stress data shows that the von Mises failure criterion is not applicable, although the departure is small except at the larger values of λ b .