Evaluation of the stress-relaxation modulus for materials with rapid relaxation rates

The stress-relaxation modulus E(t) is defined in terms of an experiment involving an instantaneously applied strain; such experiments are, in practice, impossible. We have critically investigated several methods which have been suggested in the literature to extract E(t) from experimental stress values used to bring about a constant-strain rate followed by a constant strain; such strain histories are easy to realize in the laboratory. Strengths and weakness of the various methods are analyzed in terms of data generated via an appropriate analytical expression. In addition, an alternative technique for converting experimental data to E(t), which involves a direct fit of this analytical expression to experimental data, is suggested. The accuracy of this last method has been assessed using stress-relaxation data for polystyrene in the primary (glass to rubber) transition region.