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The Free Retraction of Natural Rubber Vulcanisates
Lewis B Tunnicliffe,Alan G Thomas, James JC Busfield
Soft Matter Group, Queen Mary University of London
This paper investigates the free retraction of unfilled natural rubber vulcanisates from simple uniaxial extension. As has previously been noted1-4 the free retraction of rubber vulcanisates proceeds via an unloading pulse which traverses the material at high strain rates (1-2x103/s). The finite rate of the pulse depends on the instantaneous, incremental modulus of the material upon release.
By observing the free retraction of rubber strips using a high speed camera, the process can be quantitatively analysed. A momentum-based theory derived by Stevenson and Thomas5 for free retraction from biaxial extension is modified for the uniaxial retraction case considered here. A series of NR sulphur vulcanisates with three different crosslink density levels are released from quasi-static loadings at extension ratios from λ = 2 to λ = 5. Good agreement is found between experiment and the momentum-based model for the unloading pulse velocity when the strain rate of retraction is below ~1x103 /s. However when material retraction velocities exceed this value, significant departure from the model is observed along with a dissipation of the pulse along the length of the sample strip.
This is attributed to the non-linearity of the high strain rate retraction stress-strain curves for these materials arising from finite extensibility and strain induced crystallisation effects. By holding samples at λ = 5 for 1 hour prior to release, SIC is promoted and from the resulting data is shown to dramatically reduce the retraction pulse velocity.
Finally by considering the energetics of the retraction process in terms of the initial energy stored in the material from quasi-static loading and the kinetic energy of retraction, the hysteresis of retraction can be calculated. The hysteresis is shown to be rate dependent and dependent on the crosslink density of the NR vulcanisates.
The potential for using FEA to model these retraction processes based upon experimentally determined high strain rate retraction stress-strain curves is highlighted.
1. B.A. Mrowca, C.S.C. Dart, E. Guth, Phys. Rev., 66, 30 (1944) and B.A. Mrowca, C.S.C. Dart, E. Guth, Phys. Rev., 66, 32 (1944)
2. P. Mason, Proc. Royal Soc. A, 272, 315 (1963)
3. A.N. Gent, P. Marteny, J. Appl. Phys., 53, 6069 (1982)
4. R.B. Bogoslovov, C.M. Roland, J. Appl. Phys., 102,063351 (2007)
5. A. Stevenson, A.G. Thomas, J. Phys. D Appl. Phys., 12, 2101 (1979)