Читать книгу The Rheology Handbook - Thomas Mezger - Страница 55

When measuring at shear rates of γ ̇ < 1 s-1, it is important to ensure that the measuring point duration is long enough. This is especially true when testing highly viscous and viscoelastic samples at these low-shear conditions. Otherwise start-up effects or time-dependent transition effects are obtained, i. e., values of the transient viscosity function will be determined instead of the desired constant value of steady-state viscosity at each measuring point. Steady-state visco­sity is only dependent on the shear rate (or the shear stress) applied, resulting point by point in the viscosity function η( γ ̇ ) or η(τ), respectively. The values of transient viscosity, however, are dependent on both, shear rate (or shear stress) and passing time. Therefore, they are presented in the form of η+( γ ̇ , t) or η+(τ, t), respectively.

When performing tests at γ ̇ > 1 s-1, only samples with pronounced viscoelastic properties are still influenced by transient effects. Therefore here, for liquids showing low or medium viscosity values, a duration of t = 5 s is sufficient for each single measuring point in almost all cases. However, transient effects should always be taken into account for polymers when measuring at shear rates of γ ̇ < 1 s-1 (i. e. in the low-shear range).

As a rule of thumb: The measuring point duration should be selected to be at least as long as the value of the reciprocal shear rate (1/ γ ̇ ).