Creep rate, anisotropy

The low creep rates for the oriented material in Fig. 8 indicated that the easy shear process which produced the high anisotropy of modulus occurr before the start of the creep measurements. This is consistent with the dynamic mechanical data of Stachurski and Ward on similar cold drawn LDPE sheets, which showed a highly anisotropic relaxation, attributed to shear parallel to the oriented crystalline chains, at a frequency of 50 Hz and temperature of 0°C. From the above it would be expected that high creep rates would be obtained on specimens cut at 45° to the fibre axis, when the temperature is lowered into the relaxation region. Baker and Darlington (1973, to be published) have confirmed that such high creep rates are observed at times between 5 and 2 x 10 s, when the temperature is lowered to — 10°C. 

At low draw ratios, the tensile compliances (as distinct from the lateral compliances) exhibited appreciable time-dependence, with a marked anisotropy in their rate of change with time. As the draw ratio increased, the time dependence, and its anisotropy, decreased until, at very high draw ratio, the low creep rates presented earlier were obtained with no significant anisotropy of time-dependence. Data at draw ratios of 1-4 and 20 are presented in Figs. 10 and 11 (from Qayton ). 

Oriented thermoplastics can show large anisotropy in creep behaviour, expecially in partially crystalline polymers. Significantly different patterns of behaviour occur in different materials. Not only is there anisotropy of isochronous stiffness, but also of creep rate and non-linearity. If stiffoess is regarded as a function of time, direction and stress or strain, the behaviour is such that the variables are not normally separable. 

In terms of the strain-hardening modulus, this has been developed by the use of Kuhn and Griin models and Kratky models to relate the development of molecular orientation and meehanical anisotropy (see Section 8.6.3). With regard to the strain rate sensitivity the strain rate-dependent viscosity has been developed by studies of creep and yield behaviour (see Sections 11.3 and 12.5.1). 

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