Linear viscoelastic measurements are often taken for granted. Many users are not aware that systematic errors such as instrument compliance may significantly corrupt the test results, especially at high frequency when the sample becomes stiff in comparison to the instrument itself. Instrument compliance effects can be demonstrated, when running high modulus materials in different size plate geometries. Figure 1 shows the complex modulus of PIB measured at -20°C using 8mm and 25mm plates. With increasing frequency, the complex modulus of the test run with 25mm plate geometry deviates significantly from the result obtained with the 8mm plate geometry. In the same graph, the measured sample deformation is plotted versus frequency. While the strain amplitude for the test performed with 8mm parallel plates is virtually constant and close to the 2% command value, does the actual sample strain for the tests performed with 25mm plates, decrease with frequency. This shows that for these tests, the deformation amplitude applied by the actuator is not fully transferred onto the sample. The smaller the sample strain in relation to the command strain, the higher the compliance effect of the instrument.
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