Nonlinear viscoelastic and viscoplastic behavior of PET-based multi-layer polymeric films used in super-pressure balloon envelopes

The envelopes of super-pressure balloons developed by the French space agency (CNES) are made of a multilayer polymeric film that exhibits considerable viscoelastic and viscoplastic behavior, both depending nonlinearly on stress. A model is presented that takes into account stress depending viscoelastic and viscoplastic strain response functions observed in uniaxial creep and recovery experiments. For easy numerical implementation, the strain response functions are represented by a Prony series, whose coefficients form a continuous spectrum on the logarithmic retardation time scale. The spectrum is characterized by three stress dependent parameters: center, width, and intensity. A continuous threshold function is proposed that approximates well the experimentally observed stress dependence of the intensity parameter. The other parameters follow the same function of stress, as is confirmed by the very good agreement between model results and experimental data.