Large lobed superpressure balloons provide an enabling technology for high-altitude, long-duration flights for conducting scientific experiments. High-altitude balloons have closed envelopes filled that remain pressurized at all times by a ligher than air gas (helium), therefore the film material that constitutes the envelope is constantly stressed, especially at regions where different mechanical parts (e.g. end-fittings, tendons, envelope) are joined.
The linear low density polyethylene (LLDPE) used in NASA stratospheric balloons is a viscoelastic soft mambrane that exhibits large deformation as well as highly rate and temperature dependent response, thus a robust and stable finite-strain thermoviscoelastic constitutive model is needed to correctly predict the behavior of the material during flight, preventing its failure. Starting from full-field deformation measurements of creep compliances, uniaxial tension and biaxial inflation tests, obtained with digital image correlation technique, a model fitting has been numerically set up to obtain the material parameters. The model is based on the Boltzmann superposition principle, time-temperature superposition principle for linear viscoelasticity and a modified free-volume theory of nonlinear thermoviscoelasticity, where mechanical dilatation and distorsion of the soft membrane have been taken into account. Furthermore, the film is modeled as an orthotropic membrane under extended plane stress conditions, so that it was possible to estimate material parameters related to thickness direction.
The material model has been integrated with finite element computations through ABAQUS (Dassault Systèmes) subroutine UMAT and has been experimentally validated against data obtained from uniaxial long-duration tension tests, uniaxial cyclic tension tests and biaxial long-duration bubble tests, showing a very good agreement between numerical predictions and experimental measures over a wide range of temperatures (from room temperature to -50°C) and strain rates (from 0.001%/s to 0.1%/s).[1-2]
Lastly, a wrinkling criterion has been integrated into the finite-element model to capture the orthotropic wrinkling behavior of the thin film. 
A current research is focused on the yield locus of LLDPE material in order to correctly predict the yielding point of the membrane over different temperatures and strain rates. A viscoplastic model will be integrated in the current nonlinear viscoelastic constitutive model to predict the behavior of the material after unrecoverable deformations.
- Li, J., Kwok, K., and Pellegrino, S. (2015). Thermoviscoelastic models for polyethylene thin films. Accepted for publication in Mechanics of Time-Dependent Materials. (pdf)
- Kwok, K., and Pellegrino, S., (2011). Large strain viscoelastic model for balloon film. 11th AIAA ATIO Conference, 20-22 September, Virginia Beach, AIAA-2011-6939. (pdf)
- Gerngross, T., and Pellegrino, S. (2009). Anisotropic viscoelasticity and wrinkling of superpressure balloons: simulation and experimental verification. AIAA Balloon Systems Conference, 4-7 May 2009, Seattle, WA, AIAA-2009-2815. (pdf)