Time-dependent Yielding and Residual Strain Accumulation of Polymer Membranes

Polymer membranes are used extensively in large lightweight structures applications such as superpressure balloons. Design of these structures for long-term operations requires accurate modeling and characterization of the permanent deformation appearing in the structures due to the time and temperature-dependent properties of polymers. This work presents analytical, numerical, and experimental investigations of the onset of residual strains of a viscoelastic polymer membrane subjected to creep loading at various temperatures. The aim of this study is to establish a model that can be employed to determine the time-to-yield given the stress and temperature history a polymer membrane experiences. Two possible time-dependent criteria, the free energy criterion and the kinematic criterion are formulated for the material and their performances are investigated with the experimental data. The experimental data consist of uniaxial and biaxial material behavior characterization and residual deformation experiments are carried out at various temperatures. Biaxial experiments are conducted with the novel inflated cone experimental setup. The inflated cone method leverages the specimen geometry to achieve a non-uniform stress field when differential pressure is applied. The resulting non-uniform strain field is captured with full-field displacement measurement methods, i.e., digital image correlation. This method accelerates the experimental data collection process. The free volume nonlinear viscoelastic model for the material is calibrated with the experimental data obtained at various temperatures and stress levels. The developed kinematic model in conjunction with the nonlinear viscoelastic model can be used to estimate the residual strains if the stress and temperature history a polymer membrane experiences are given. Similarly, the criterion can be used in an inverse design problem to limit the creep stresses and corresponding allowable durations