The influence of the finite step time on the measurement of the viscoelastic response functions

The ideal stress relaxation experiment is defined as the imposition of an instantaneous strain. In practice, it takes a finite time t₁ to reach the constant strain. Various ways in which to account for the finite step time and the subsequent effects on the relaxation modulus G(t) are examined in the present work. First, we consider the "rule of thumb" in which data are ignored until 10 times the strain application time t₁. In addition, the Lee-Knauss algorithm is compared with the Zapas-Craft method in which the corrected time of the experiment becomes t-t₁/2 where t is the experiment time. A surprising result is that the different correction schemes affect the estimates of the material parameters more than they affect the relative differences between the corrected data and the ideal behavior.