Determination of the plastic behaviour of solid polymers at constant true strain rate

Abstract

The methods of conventional tensile testing as applied to solid polymers are compared and reviewed critically. Experiments were performed using these techniques, and it is shown that large variations in local strain rate occur while necking and cold-drawing take place. A new tensile testing method is described in which the samples are tested atconstant local true strain rate. This technique is based on the use of a diameter transducer, an exponential voltage generator and a closed-loop testing machine. Flow curves for poly(vinyl chloride) and high density polyethylene were determined at room temperature over the strain rate range of 10⁻¹ to 10⁻⁴ sec⁻¹. It is shown that the flow behaviour of these two polymers can be approximated by the constitutive relation:$\sigma = K \cdot \exp [(\gamma _ \in /2) \in ^2 ] \cdot \dot \in ^m$, whereK andγ _∈ are constants andm, the rate sensitivity, is in the range 0.02 to 0.06. It is concluded that the positive curvature of the log σ flow curve is responsible for the stabilization of flow localization associated with cold drawing, and that the rate sensitivity plays a much smaller role.