The free retraction of natural rubber: A momentum-based model

Abstract

The free retraction of vulcanised strips of natural rubber released from simple uniaxial deformations is studied using high speed cinematography in the context of a simple momentum theory. A good agreement between the theory and experiment is observed when vulcanisates are released from stresses below 1 MPa which corresponds to tensile strains rates below 1x10 3 s-1. Above this critical stress and corresponding strain rate value, an increasing dispersion is observed in the form of a slowing down of the characteristic retraction pulse and also by a relaxation of strain ahead of the pulse front (a dispersion of the pulse). Holding samples at high strains for an extended period of time prior to releasing results in a further, significant retardation of the retraction pulse velocity. These effects are related to the increasing non-linearity of high strain rate retraction stress-strain behaviour. Energy balance arguments show that the dispersion of the retraction pulse is a prerequisite for pulse propagation and its magnitude underpins the deviation from the momentum model outlined in this paper.

The effect of size-scale on the kinematics of elastic energy release

“…Finally, with those same assumptions, the maximum center-of-mass velocity is determined by the product of acceleration and duration, v max = a max ∆t, yielding an expression consistent with the maximum velocity found in previous work for a linear elastic material [32] v max = c sec in .…”

“…(4). Factors that could affect the ramp-up/rampdown time include frictional losses from interaction of the band with the pneumatic clamp [32], inertia of elastomer material inside the clamp, dispersion of the elastic wave due to losses within the material or to the environment [29], and residual strain left in the band at the point of buckling [30]. These losses depend on both material properties of the band and external factors.…”

“…To understand the upper limit of elastic energy release in a resilient material system, we perform experiments measuring the high speed free retraction of a polyurethane elastomer composite with similar resilience and wavespeed to resilin. Building upon recent work [29][30][31][32][33][34], we release long thin bands of the elastomer from an initially uniform uniaxial extension, and track its displacement field on unloading. The displacement field is used to obtain the center-of-mass motion of the band, which allows for a functional determination of the scaling relations that define the limits of impulsive elastic performance.…”

Achieving High-Speed Retraction in a Stretchable Hydrogel

“…The acceleration is similar to the jump of a froghopper (≈5.4×10 m s -2 ). 5 The retraction velocity in our gel is lower than dry, vulcanized natural rubber (≈125 m s -1 ) 37 but somewhat similar to the polyurethane elastomers (≈25 m s -1 ). 5 To the best of our knowledge, this is the first demonstration of achieving such a high velocity and acceleration in hydrogels.…”

“…It accelerates from the static condition to the peak velocity resulting in curvature in the position vs. time data. 8,[37][38][39][40][41] A sharper curvature signifies a higher acceleration value. Finally, the string reaches a constant velocity displayed by a straight line with a constant slope.…”