Test MethodInvariant elastic modulus of viscoelastic materials measured by rate-jump tests

It is demonstrated that, when polyethylene is subject to a tensile test with a jump in the loading rate, an elastic modulus, given as the ratio of the jump in stress rate to the resultant jump in strain rate, is invariant with respect to the magnitude of jump in the loading rate, at a given strain at which the jump is imposed. However, such an elastic modulus measured from rate jumps applied at different strains in the polymer can be different. These observations are consistent with a constitutive description of the mechanical behaviour of the polymer in terms of a network of linear elastic springs and (in general) nonlinear viscous dashpots. During the rate jump, the dashpots are infinitely stiff and do not respond, and the response of the elastic springs in the network gives an overall elastic modulus which is invariant with respect to the magnitude of the rate jump. As the polymer is deformed to different strains, the spring-dashpot network itself may change as a result of evolution of the microstructure, hence the overall elastic modulus describing the collective behaviour of the springs in the network may change. These findings illustrate that an effective elastic modulus can be defined from a spring-dashpot network description of an (in general) nonlinear viscoelastic material, and such a modulus can be measured by a rate-jump test.