Predictions of nonlinear thermoviscoelastic properties of amorphous polymers
Abstract
A three dimensional nonlinear thermoviscoelastic model based on rational thermodynamics and incorporating a material time that depends upon the configurational entropy has been recently developed (Lustig, Shay, and Caruthers, 1995). The thermoviscoelastic model can describe many features of the mechanical behavior of polymers, including isothermal volume relaxation, nonlinear stress-strain behavior including yield, and nonlinear creep deformation. In this work, the predictions of the thermoviscoelastic constitutive equation in multiaxial deformations are presented. The predictions for uniaxial extension including loading and unloading are in reasonable agreement with experimental data for polyvinyl acetate. The model predicts the asymmetry of the yield stress exhibited by polymeric materials in extension and compression. The effects of temperature, pressure, strain rate, and physical aging on the predictions of yield stress have also been investigated and predictions agree with the experimental observation. The viscoelastic behavior and yield in biaxial deformations are also predicted by the thermoviscoelastic model, and the yield surfaces predicted by the model agree with the empirical von Mises criterion. This is significant because a three dimensional yield is not explicitly built into the constitutive equation, but rather naturally appears as a prediction of the constitutive equation. The interrelationship between the evolution of various thermodynamic properties, such as specific volume, configurational entropy, and the viscoelastic stress response of the polymer in anisotropic deformation is investigated. The thermoviscoelastic and yield behavior of an Epon 1001F/DDS epoxy resin in uniaxial deformations at different strain rates and temperatures is investigated. A limiting thermoelastic form of the more general thermoviscoelastic constitutive equation is developed for the deformation conditions well below glass transition temperature. The rheological properties at conditions well above glass transition temperature such as deformation rate dependent shear viscosity and elongational viscosity are also predicted by the thermoviscoelastic model.
Degree
Ph.D.
Advisors
Caruthers, Purdue University.
Subject Area
Chemical engineering|Materials science|Plastics
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