Predictions of nonlinear viscoelastic behavior using a hybrid approach

Abstract

An approach for nonlinear viscoelastic characterization is presented which uses the combined measurements from creep and dynamic mechanical tests. Although the methodology should extend to several materials and geometries, this research concentrates on thin film polymers used in the manufacture of high altitude scientific balloons. Typically, the constitutive behavior of these materials is characterized through the use of linear viscoelastic techniques. Although this linear approach provides an accurate model for small strains or loads, these materials have been shown to be highly stress dependent and, consequently, it is necessary to identify this nonlinear behavior. Traditional creep measurements require extensive laboratory test times, yet the results obtained from dynamic mechanical analysis provide the capability to predict long term material performance without a lengthy experimentation program. However, dynamic mechanical methods are currently limited to linear response; thus, an approach is presented in which the stress-dependent behavior is derived from short-term creep measurements in a manner analogous to time-temperature superposition. Predictions of material response using linear and nonlinear approaches are compared with experimental results obtained from traditional long-term creep tests. Although linear pre-dictions deteriorate for large stresses, excellent agreement is shown for the nonlinear model.