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1 Manual or visual determination, respectively

In practice, the aim is to produce a creep curve showing a constant slope at the end of the creep phase. In order to get a useful basis for the analysis, as a rule of thumb, at least 10 % of the measuring points of the entire creep curve should occur within this range of steady-state flow. Taking a ruler, it is useful to draw a straight line in this test interval to facilitate visual evaluation. The following results are obtained from this sector of the curve: The change in deformation Δγ read-off on the γ-axis, and the corresponding period of time Δt read-off on the time axis. The first calculation step is: Δγ /Δt; this is the deformation change in the corresponding period of time, or time-dependent rate of deformation (shear rate) γ ̇ 0. The second calculation step (see Figure 6.8):

Equation 6.3

η0 = τ0 / (Δγ /Δt) = τ0 / [Δγ3 / (t2 – t1)] = τ0 / [(γ3 – γ2) / (t2 – t1)] = τ0 / γ ̇ 0

1 Automatic determination using a software analysis program

First of all, the user defines the part of the final sector of the creep curve to be used for the characterization of steady-state behavior, i. e. when dγ/dt = const is reached. Like above, as a useful rule of thumb at least 10 % of the measuring points of the whole creep curve should occur within this steady-state sector to outline the corresponding curve interval. Then, the straight analysis line is fitted in this curve sector by the software program, finally determining also the value of η0.