  Four series of tensile loading-unloading tests are performed on isotactic
polypropylene in the sub-yield domain of deformations at room temperature. In
the first series, injection-molded specimens are used as produced, whereas in
the other series the samples are annealed for 24 h at 120, 140 and 160 C, which
covers the low-temperature region and an initial part of the high-temperature
region of annealing temperatures. A constitutive model is developed for the
elastoplastic behavior of a semicrystalline polymer. The stress-strain
relations are determined by five adjustable parameters that are found by
fitting the experimental data. The effect of annealing is analyzed on the
material constants.
