Uniaxial and biaxial stretching of silicone putty

Two novel experimental methods are used. Vertical uniaxial stretching is obtained by attaching a perspex rod to the lower end of a silicone putty cylinder; the rod then descends into water of constant depth. The stress and rate of extension change little during each test, but the rate of extension may be varied from 0.005 to 0.10 s⁻¹ by modifying the experimental conditions. Biaxial stretching is acchieved by placing a disc of silicone putty across the top of an open glass cylinder which is lightly pressurized. The sample expands as a spherical cap, the height of the centre above the cylinder being timed. The stress in the cap passes through a shallow minimum as it expands (at constant pressure) and the slowly varying rate of biaxial extension may be readily determined. This lies in the range 0.003–0.06 s⁻¹. For low rates of uniaxial or biaxial extension, it is possible to plot the extension against time and to show how the extensional viscosity varies with the strain rate (or principal extension ratio). For high rates of extension, a ‘single point’ determination of the extensional viscosity may be made, with the stress and strain rate averaged at the mid-point of the sample's extension. The temperature is 26.5 ± 1.5 °C. The following is shown under the experimental conditions:

(a) the extensional viscosity (uniaxial or biaxial) is in the range 1.0 × 105 to 3.0 × 10⁵ Pa s;

(b) for extensional strain rates between 0.01 and 0.04 s⁻¹, the uniaxial and biaxial extensional viscosities are of comparable value;

(c) both forms of the extensional viscosity tend to decrease with increased extensional strain rate, the biaxial extensional viscosity falling more rapidly and being higher than the uniaxial viscosity at low strain rates and lower at high strain rates;

(d) there are no signs of rupture in uniaxial extension (principal extension ratios up to 1.8 and extensional strain rate up to 0.1 s⁻¹);

(e) in biaxial extension, the sample tends to rupture more easily as the strain rate is increased. (The sample fails at the principal extension ratio of 2.0 at an extensional strain rate of 0.02 s⁻¹ and fails at a principal extension ratio of 1.3 at an extensional strain rate of 0.07 s⁻¹.)