The behavior of polymers is significantly influenced by hydrostatic pressure. In this paper, the effects of hydrostatic pressure and the third invariant of the deviatoric stress tensor on the nonlinear visco-elastic deformation of celluloid are first discussed in connection with experiments of torsion of tubular specimens and tension of uniaxial specimens. The distinctive features of the deformation behavior in creep and for proportional loading (namely, uniform rate of stress increase with time) under superimposed hydrostatic pressure are investigated by using the nonlinear visco-elastic celluloid. The stress-strain relation of the nonlinear visco-elastic media with regard to the effect of the hydrostatic pressure is deduced from the invariant theory with an hypothesis of creep potential. The deduced relation gives fairly good agreement with the actual observations.

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