A freely expanding ring technique for measuring the tensile properties of polymers

Abstract

The measurement of the stress-strain properties of materials at high strain rates is principally carried out using the split Hopkinson pressure bar (SHPB) technique, in which high compressive strain rates, typically in the range 10 2 -10 4 s −1 , are achievable with small solid discs as the test materials. Torsional SHPB systems have also been developed for test specimens in the form of small hollow cylinders. Another version of the SHPB technique has enabled high-strain-rate tensile measurements to be made, but inertial effects tend to limit the strain rate to about 10 3 s −1 for the dogbone-shaped specimens. To achieve higher tensile strain rates, the authors have developed an expanding ring technique. This involves placing a thin ring of polymer as a sliding fit around a hollow, thick-walled, polymeric cylinder, the internal wall of which is loaded with a blast wave generated by an exploding wire. The tensile stress-strain properties of the ring material have been determined, at strain rates up to 1.6 × 10 4 s −1 , by observing the movement of the ring as it freely expands away from the cylinder. Several polymers have been studied and comparisons made with compressive stress-strain behaviour.

The high strain rate compressive behaviour of polycarbonate and polyvinylidene difluoride

“…This confirmed that the drop in strength observed by Walley and Field was not inconsistent with the bilinear behaviour in other studies on the same materials. In addition, use of the expanding ring technique showed that this behaviour is not only observed in Hopkinson bar experiments [15].…”

High Strain Rate Mechanics of Polymers: A Review

“…In addition, it is necessary to overcome the effects of the inertia of the apparatus, so that high speed deformations can be applied after a very short period of acceleration. Hydraulic machines are often used; however, systems based on dropping weights [39][40][41][42][43][44][45][46][47][48], fly wheel systems [49,50], expanding ring [51], cam plastometer [52], very long Hopkinson bars [53], or the 'wedge bar' [54] have also been applied successfully. Accurate experiments in this strain rate regime are key because molecular mobility transitions often become activated between 1 and 1000 s -1 .…”

Mechanical Behavior of Rubber at High Strain Rates

“…The walls of the cylinder are rapidly expanded using either an electromagnetic pulse 95 or gas pressure generated inside the cylinder by an explosive charge or rapid vaporization of a wire subjected to a sudden high voltage. 96,97 The cylindrically symmetric expansion of the walls transfers momentum to the ring, causing it to move away from the tube. The deformation state of the sample is uniaxial tension provided the ring thickness is no more than 5% of its diameter.…”