Viscoelastic effects on the free retraction of rubber

Abstract

Wave propagation and retraction velocities were measured for two elastomers, a 1,4-polybutadiene and a polyurea, freely retracting from large tensile strains ͑ഛ2͒. From these data the stress-strain response was calculated. The achievable strain rate depends on the initial strain and the viscoelasticity of the material, with values exceeding 1800 s −1 attained herein. Thus, the method can be used to characterize the mechanical behavior at high strain rates, as well as high strains. A drawback is that the strain rate is not constant during the retraction. The kinetic energy of retraction reflects the unrelaxed stress, providing a straightforward determination of strain energy and its dissipation. The two elastomers represent extremes of viscoelastic behavior, as reflected in the retraction response at both low and high strain rates.

On the response of rubbers at high strain rates.

“…These follow along the lines of Mrowca et al (1944), Mason (1963) and Bogoslovov and Roland (2007) who conducted experiments involving the free retraction of rubber from a prestrain. We use the power-law model and shock jump conditions to examine free-retraction experiments.…”

“…This requires performing numerical differentiation of the measured particle position to get velocity and strain and then differentiating again; this process is subject to large errors and cannot provide reliable estimates of the stress-strain response. Moreover, Bogoslovov and Roland (2007) also did not observe shock waves; they suggested incorporating a viscous term to account for dissipation while extracting the unloading stressstrain curve. An examination of the particle trajectory diagram shown by Mason (1963) as well as Bogoslovov and Roland (2007) reveals clearly that the particle speed changes abruptly from zero to its final value, immediately upon the arrival of what they label as the unloading wave; this should really correspond to a shock wave.…”

“…Moreover, Bogoslovov and Roland (2007) also did not observe shock waves; they suggested incorporating a viscous term to account for dissipation while extracting the unloading stressstrain curve. An examination of the particle trajectory diagram shown by Mason (1963) as well as Bogoslovov and Roland (2007) reveals clearly that the particle speed changes abruptly from zero to its final value, immediately upon the arrival of what they label as the unloading wave; this should really correspond to a shock wave. In this report, we explore the propagation of stress waves upon unloading or freeretraction.…”

“…More recently, Bogoslovov and Roland (2007) considered the same free retraction problem in two different elastomers: 1:4 polybutadiene and a polyurea. Essentially, they used Mason's analysis to interpret their experiments.…”

Comparison of the transient stress–strain response of rubber to its linear dynamic behavior

“…50 m/s) were observed at the beginning of loading (e < 0.1), similar to effects described elsewhere. 38,39 The strain rate results reported here were determined at higher strains and do not include these transient effects.…”