Loading-Unloading-Recovery Curves for Polyester Yarns and Identification of the Nonlinear Maxwell-Type Viscoelastoplastic Model

The paper presents results of uniaxial tensile tests of a multifilament polyester yarn in three-stage loading and unloading programs with different loading rates and subsequent recovery to study its viscoelastoplastic properties. The verificatio n of applicability of a physically nonlinear constitutive relation of Maxwell-type viscoelasticity with two material functions, studied in detail earlier, was accomplished by means of loadingunloading- recovery curves with different loading rates and durations (in the range up to 70% of the tensile strength of the yarn). At the first stage of an express inspection, it was established that most of the basic applicability indicators were fulfilled with an acceptable accuracy. However, one indicator was not fulfilled and warned us about possible difficulties in identifying and applying the Maxwell model to the describing the behavior of the polyester yarn. Two material functions for the multifilament polyester yarn were determined and two methods for identifying the constitutive relation according to the loading-unloading-recovery curves were developed. The ways to improve the initial identification methods were indicated. A fast and economical identification technique using only one loading-unloading-recovery curve was proposed that describes behavior of the polyester yarn more accurately. The material function describing the elastic strain was determined with a high accuracy. It appeared to be linearly dependent on the stress of the polyester yarn on the entire stress range considered. Verification of the calibrated model with the material functions determined carried out in several ways. It demonstrated that the model described satisfactorily the test data for the polyester yarn in the complex test programs. It was shown that an improved approximation of the viscoplastic strain by the model can be achieved by using tests with different loading durations. It was also shown that, for the polyester yarn, a series of tests with a loading duration of 300 s and various loading rates is insufficient, and the loading time has to be at least 900 s.