Study of Time-Dependent Properties of Thermoplastics

Abstract

SummarySimple tests carried out with a common tension/compression testing machine are used to obtain timedependent properties of non-reinforced thermoplastics. These tests include ramp loadings as well as relaxation and creep tests. Two materials (PBT Celanex 2002-2 and POM Hostaform C9021, Ticona GmbH, Kelsterbach) were taken for the experiments. The experiments show that an adequate description of the long-term material properties can be obtained from the short-time tests, namely from tests with constant traverse speedL.Below a model for the time-dependent mechanical behavior is presented and fitted to the obtained measured data. For the evaluation of the fitting quality long-term tests are used. Especially creep and relaxation tests with "jumps", i.e. rapid change of loading, are important for this purpose. ExperimentsThe following experiments were carried out: ramp loading with constant stress rateσ = const and with constant traverse speedL = const, standard creep and relaxation tests as well as creep and relaxation tests with rapid change of loading ("jumps"). The ramp loading tests are short-time ones, the remaining tests are long-time. The deformations occurring during the tests are captured on a suitable element of the specimen's surface using the optical measuring system Vic-2D, Limess Messtechnik & Software GmbH, Pforzheim.The ramp loading tests are simple and with the settingL = const relatively stable. It is shown that these tests are sufficient to obtain the parameters of the Bailey-Norton functioṅthe elastic modulus E as well as the parameter α, which describes the material response to "jumps". The mentioned properties are required to fit the model to the measured data. If the material properties should be obtained from the long-time experiments, so the creep test only is not sufficient, and relaxation test is required (Fig. 1). The tests with "jumps" are essential in order to capture the material properties. A test with three "jumps" is used in order to control the quality of fitting. Since the strain is measured in both the longitudinal and the transverse directions, it is possible to compute the Poisson's ratio [2]. This can be used as an additional verification for the model.