Utility of Elasticity Theory

A brief outline of the extension of the formalism of the classical theory of elasticity for the description of non-linear viscoelastic behaviour was 

A high degree of anisotropy of both isochronous compliance and nonlinearity of behaviour is found in highly drawn LDPE having fibre symmetry, as shown by Raumann and Saunders and Darlington and Saunders and thus provides a suitable system in which to examine the utility of the theory. 

The variation of the isochronous modulus at 100 s with the magnitude of the creep strain at 100 s for strains in the region OT-10% in samples cut at various angles to the fibre axis is shown in Fig. 3. The data were obtained using the isochronous stress-strain procedure, previously referred to, on LDPE drawn at 20 C so as to produce fibre symmetry with a draw ratio of 4-2. In this figure horizontal straight lines would indicate linear viscoelastic behaviour. The strain at which significant deviation firom 

In Fig. 4 cross-plots at tensile strains of 0 2% and 5% are presented as points (with some additional points). The full lines in this figure are the theoretical lines derived from eqn. (7) fitted, in the case of 0-2% strains, at = 0, 45 and 90° and in the case of 5% strain at 0 = 0,45 and 80 . No experimental value of (90) was available above a tensile strain of 1-7% owing to early fracture being a feature of behaviour in this direction. 

It is apparent from the agreement between the theoretical lines and the points for the remaining values of 0 that the angular variation of modulus suggested by eqn. (7) applies over a wide range of tensile strains. 

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