Intrinsic strain softening appears to be the main cause for the occurrence of plastic localization phenomena in deformation of glassy polymers. This is supported by the homogeneous plastic deformation behavior that is observed in polycarbonate samples that have been mechanically pretreated to remove (saturate) the strain softening effect. In this study, some experimental results are presented and a numerical analysis is performed simulating the effect of mechanical conditioning by cyclic torsion on the subsequent deformation of polycarbonate. To facilitate the numerical analysis of the mechanical rejuvenation effect, a previously developed model, the compressible Leonov model, is extended to describe the phenomenological aspects of the large strain mechanical behavior of glassy polymers. The model covers common observable features, like strain rate, temperature and pressure dependent yield, and the subsequent strain softening and strain-hardening phenomena. The model, as presented in this study, is purely single mode' (i.e., only one relaxation time is involved), and therefore it is not possible to capture the nonlinear viscoelastic pre-yield behavior accurately. The attention is particularly focused on the large strain phenomena. From the simulations it becomes clear that the preconditioning treatment removes the intrinsic softening effect, which leads to a more stable mode of deformation.

The Influence of Intrinsic Strain Softening on Strain Localization in Polycarbonate: Modeling and Experimental Validation

Mechanical behavior and failure mechanisms of Li-ion battery separators

Long-term failure under constant or cyclic load is governed by the same process as short-term failure at constant rate of deformation. Failure proves to originate from the polymer’s intrinsic deformation behavior, more particularly the true strain softening after yield, which inherently leads to the initiation of localized deformation zones. In a previous study we developed, and validated, a 3D constitutive model that is capable to predict the occurrence of these plastic instabilities, yielding quantitative predictions of the lifetime of polycarbonate under constant load.(1) Here we demonstrate that the same approach is also applicable to predict the life span of polycarbonate under cyclic loading conditions, over a large range of molecular weights and thermal histories, with a single parameter set only. The model incorporates the influence of physical aging, accelerated by the applied cyclic stress. For low cycle fatigue, at large stress amplitudes, where failure is thermally dominated, it is shown that ...

http://www.mate.tue.nl/mate/pdfs/8836.pdf

Fatigue Life Predictions for Glassy Polymers: A Constitutive Approach

The science and technology associated with the mechanical properties of thermoplastics still harbour many mysteries. There are numerous different aspects of mechanical properties and their evaluation entails many types of measurement, such as short term modulus and creep, yield behaviour and large deformation after yield, toughness and fracture. In various ways all these measurements are related to each other, but until about 1970 the importance of large strains in determining other mechanical properties, often measured separately, was not generally appreciated. Since that time much progress has been made, though there are still many uncertainties concerning the extent and manner in which different mechanical properties are connected. It is therefore appropriate, in a review of large deformations in glassy polymers, to begin by summarizing briefly the way in which ideas on the interrelations between mechanical properties have developed and why it is that large strains are of particular interest. This is also of importance in efforts to understand the differences responsible for tough and brittle fracture.

The post-yield deformation of glassy polymers

The produced mechanical work at tensile drawing is converted into heat almost completely and the polymer temperature rises. The temperature rise at adiabatic drawing for different polymers is 20-100°C. To determine the temperature of PET during neck propagation, micro-particles with pre-determined melt surface were dusted on the sample surface. If particles melted, surface temperature exceeded the particle melt temperature. In addition, the temperature rise was analyzed theoretically. The surface temperature of 140°С of PET film samples was recorded. Equations describing distribution of temperature are derived. The equations are solved for steady neck propagation. The drawing ratio in the neck and the draw stress are the main parameters determining the temperature rise. A new method of measurement of the heat transfer coefficient from the neck temperature profile is developed. When the temperature in the yield zone reaches the glass transition temperature, appear pores, the neck drawing ratio increases, and further temperature rises are observed.

/pdf/non-adiabatic-heating-of-polymer-films-under-drawing-2fpr6plhko.pdf

Non-adiabatic heating of polymer films under drawing

Observation of the heat exchange during deformation using an infra-red camera

Quantitative measurement of the heat exchange during deformation using an infrared camera

The simple measurement of birefringence by means of a tilting compensator becomes difficult and unreliable when large phase differences must be compensated in samples in which dispersion of birefringence differs from that of the compensator. The problem is treated experimentally and theoretically. The result is a simple correction formula with only one parameter. A method to obtain this parameter is presented. Measurements on polycarbonate, polyethylene and polyurethane support the assumptions of the theoretical calculations.

Joachim A. Koenen

Papers

Observation of the heat exchange during deformation using an infra-red camera

Quantitative measurement of the heat exchange during deformation using an infrared camera

Determination of the compensation band due to birefringence with dispersion and large phase differences