Читать книгу The Rheology Handbook - Thomas Mezger - Страница 143

When presetting low shear rates or shear stresses, the duration for each individual measuring point must be long enough. Otherwise particularly with highly viscous and viscoelastic samples, time-dependent start-up effects or transient effects are obtained. In this case, the measured curve approaches the final value of τ or η from below (see also Chapters 3.3.1b and Note 1 in and 3.4.2.2a). When applying high shear rates, viscous shear-heating may occur to a higher extend and then, the time-dependent viscosity values may decrease additionally due to thermal effects. In this case, with low-viscosity liquids also turbulent flow effects must be taken into account (see also Chapter 3.3.3).

Extended test profiles (including intervals at rest, for temperature equilibration, and pre-shearing)

Sometimes, test programs are used showing several intervals with preset of a constant shear rate in each; for example:

1 Rest intervals, presetting constantly γ ̇ = 0, either at the start of the test to enable relaxation of the sample after gap setting of the measuring system which may cause high internal stresses particularly when testing highly viscous and viscoelastic samples. Simultaneously, this period is suited to enable temperature equilibration, or after a pre-shear interval.

2 Pre-shear intervals , presetting a constant low shear rate, to ensure a defined sample preparation, to distribute the sample in the shear gap homogeneously, and to equalize or to reduce pre-stresses deriving from diverse sample preparation steps.