Enlarged peaks
Internal friction measurement

The model of the standard Zener viscoelastic solid, which we have presented as an example, corresponds to an elementary mechanism with a single relaxation time. In reality, in most cases, the observed relaxation peaks are wider than a Debye peak.

In this case, we consider that these peaks are composed of several juxtaposed peaks, or that there is a continuous relaxation time spectrum; in this case, it is necessary to introduce a normalized relaxation time distribution function.

Nowick and Berry [2] introduced a Gaussian distribution, based on the statistical distribution of relaxation times around a most probable mean value. Other authors have used a Fuoss-Kirkwood distribution, which has the advantage of an analytical solution.

Recently, it has been...