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Data of a creep test are available in terms of a γ(t)-function. The aim is to determine the

corresponding frequency function (oscillatory tests). The following steps are performed:

Using available data of the creep function γ(t)

 Calculation of the function of the creep compliance J(t)

 Calculation of the continuous retardation time spectrum H(Λ)

 Data conversion to determine the corresponding frequency sweep in terms of G’ & G’’(ω)

This method is particularly useful if it is aimed to produce values in the range of zero-shear ­viscosity (or low-shear range, resp.).

Information given in Chapter 7.3.4 on data conversion and on curve fitting methods also applies here, in the appropriate form, to creep test data. Data conversion can be performed using data of both the time-dependent relaxation modulus G(t) or creep compliance J(t). Relaxation spectrum H(λ) and retardation spectrum H(Λ) correspond to each other approximately. However, it should be taken into account that data which are depending on relaxation times λ or retardation times Λ, respectively, are measured at different shear conditions: for data related to λ with controlled strain (deformation) γ, and for data related to Λ with controlled stress τ. There might be a different response by the structure of the sample when subjected to these different shear conditions.