The beating effect in uniaxial oriented polymer materials

Elastic-relaxation properties of uniaxial oriented polymer materials under dynamic deformation mode are investigated. A theoretical explanation of the phenomenon of the occurrence of beats is proposed. Experimental confirmation of the obtained theoretical results is given. Using the barrier theory and applying the balance equation of the number of cluster transitions through the energy barrier, taking into account the transition time, a refined constitutive equation of the viscoelasticity of the polymer materials is obtained. Experimental studies in the dynamic mode were carried out by the method of the longitudinal low-amplitude oscillations. For a polymer material, taking into account the time of transition of the cluster through the energy barrier, a refined constitutive equation is obtained. The resulting equation is a second-order differential equation and admits a periodic solution. The application of the obtained equation to the study of free longitudinal low-amplitude oscillations in uniaxial oriented polymer materials is considered. It is shown that the solution of the equation admits two close complex roots, the existence of which leads to the effect of the observed beats. The relations between the parameters of the oscillatory process and viscoelastic characteristics are obtained. The experiment showed that the polymer materials under study have a complex form of the oscillatory process, similar to the phenomenon of beats, in a certain range of load. The dependence of the tangent of the mechanical losses angle calculated from the main frequency on the stress has an acute maximum in the stress range where beats are observed. According to the obtained theoretical ratios and experimentally determined attenuation coefficient and main frequency, the time of transition of the cluster through the energy barrier or the lifetime of the cluster in this energy state is determined. The use of additional information obtained during experiments in static modes made it possible to determine the relaxation time for this material. The calculation on the example of polyethyleneterephthalate (PET) filament showed that the theory is consistent with the experiment. Based on the experimental data obtained during the study of free longitudinal oscillations and the solution of the refined constitutive equation, it is possible to determine the necessary viscoelastic characteristics which makes it possible to predict dynamic deformation processes in polymer materials.