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Yield point values depend on the duration of the test. With each new measuring point at the beginning of each new step on the stress ramp, the structure of the sample is stretched at first under the applied constant shear load. As a consequence, a constant, steady-state measuring value is resulting (e. g. in the form of the shear rate γ ̇ ) but only after a certain delay. To avoid this time-

dependent start-up effect which is also called transient effect, the user should wait sufficiently long at each measuring point (see also Chapter 3.3.1b and Figure 2.9: no. 5). For samples showing clearly time-dependent behavior, differing measuring times for an otherwise identical preset test profile may result in different yield point values. A yield value also depends on the sample preparation before the test (e. g. concerning shear load, time effects, temperature) [3.26].

Summary: A yield point is not a material constant. Since it is time-dependent, it depends on the conditions during the preparation of the sample as well as on the test conditions.

Note: Structural strength at rest and frequency sweeps

When determining structural strength or consistency-at-rest, frequency sweeps (oscillatory tests) are the better way of testing in principle since they take best into account the influence of time. Here, when presetting frequencies, time-dependent results are obtained since a frequency is an inversed time (see also Chapter 8.4.4a).

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