Nonlinear thermomechanical response and constitutive modeling of viscoelastic polyethylene membranes
Thin films of linear low-density polyethylene show a significant time-dependent behavior, strongly reliant on temperature and strain rate effects. A constitutive nonlinear thermo-viscoelastic relation is developed to characterize the response of thin membranes up to yielding, in a wide range of temperatures, strain rates, and mechanical loading conditions. The presented plane stress orthotropic formulation involves the free volume theory of viscoelasticity and the time-temperature superposition principle, necessary to describe non-linearities and non-isothermal conditions. Uniaxial tension tests at constant strain rate and long-duration biaxial inflation experiments have been employed in the calibration of the material parameters. The model has been implemented in the Abaqus finite element code by means of a user-defined subroutine based on a recursive integration algorithm. The accuracy of the constitutive relation has been validated against experimental data of full field diaphragm inflation tests and uniaxial tension, relaxation and cyclic experiments, covering sub-ambient temperatures and strain rate ranges observed during the operation of stratospheric balloons.
© 2017 Elsevier Ltd. Received 3 March 2017, Revised 9 October 2017, Accepted 10 October 2017, Available online 12 October 2017. We thank Prof. W.G. Knauss (Caltech), Dr. David Wakefield (Tensys Limited) and Dr. James Rand (Winzen Engineering) for helpful comments and discussions. Financial support from the NASA Balloon Program Office is gratefully acknowledged.