The effect of annealing on the elastoplastic response of isotactic polypropylene

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 degreesC, 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 elasto-plastic 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. (C) 2002 Elsevier Science Ltd. All rights reserved.