Maxwell model stresses observed experimentally

A stress relaxation experiment can be performed on a wide range of materials. If we perform such a test on a real material a number of deviations are normally observed from the behaviour of a single Maxwell model. Some of these deviations are associated with the application of the strain itself. For example it is very difficult to apply an instantaneous strain to a sample. This influences the measured response at short experimental times. It is often difficult to apply a strain small enough to provide a linear response. A Maxwell model is only applicable to linear responses. Even if you were to imagine an experiment where a strain is... 

It can be seen that the first 2 terms in the first square brackets correspond to the Maxwell element response and the other terms correspond to the Kelvin element response. The multiparameter model describes the response observed for rPET polymer concrete under a constant load. To obtain the creep strain using Equation 4.33 for a given stress, it is necessary to compute multiparameters, which are the 3 elastic constants ( i, 2 and 3) for the springs and the 3 viscous constants rp, % and 7/3) for the dashpots. Eigure 4.14 shows graphically how to obtain these values from the experimental results. 

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