  A series of tensile relaxation tests is performed on isotactic polypropylene
in the sub-yield and post-yield regions at room temperature. Constitutive
equations are derived for the time-dependent response of a semicrystalline
polymer at isothermal loading with small strains. Adjustable parameters in the
stress-strain relations are found by fitting experimental data. It is
demonstrated that the growth of the longitudinal strain results in an increase
in the relaxation rate in a small interval of strains in the sub-yield domain.
When the strain exceeds some critical value which is substantially less than
the apparent yield strain, the relaxation process becomes strain-independent.
