Anisotropic Viscoelasticity and Wrinkling of Superpressure Balloons: Simulation and Experimental Verification

Abstract

Super-pressure balloons are currently under development by the NASA Balloon Program Office for use in stratospheric balloon missions. They are made of thin polyethylene film forming a sealed envelope that is contained by stiff meridional tendons. The film is subject to a state of stress whose details depend on the cutting pattern, stiffness of the film vs. stiffness of the tendons, etc. and the viscoelastic behavior in the film plays an important role. This paper extends the modeling approach presented by the same authors at the AIAA Balloon Systems Conference 2007. The current model captures nonlinear viscoelasticity in a wrinkled, anisotropic membrane, and the analysis has been successfully applied to several balloon designs. Because the effects of wrinkling on the stress history are modeled correctly, the behavior of the balloon can be simulated with high fidelity, starting from pressures as low as 5-20 Pa. The creep strains at selected points of a 4 m diameter balloon were measured using photogrammetry and the results were compared to results from the numerical model. At a pressure of 700 Pa the balloon had a maximum stress of 6.0 MPa in the meridional direction (2.5 MPa hoop stress) and meridional strains of up to 6.4% (-2.7% hoop strain). A detailed model of gore seams and tendon attachments provides insight into the asymmetry of the strain distribution that results from the actual asymmetry of the gore seams. This also allows the numerical replication of an experimentally observed pressure-dependent rotation of the end-fittings.

Non-Linear Viscoelastic Analysis and the Design of Super Pressure Balloons: Stress, Strain and Stability

“…An alternative implementation by Gerngross 13 assumes that the stress function Θ varies linearly over the current time interval. Comparison for a number of test problems has shown no significant differences between the two approaches.…”

Large Strain Viscoelastic Model for Balloon Film

“…[4][5][6] A biaxial model for StratoFilm 420 based on the Schapery model was formulated by Rand and Sterling 3 and implemented in a finite element simulation to study the overall stress distribution in a superpressure balloon. 7 Recently, applicability limits of the Schapery model were established by comparing the predictions from the model to biaxial tensile strength tests. 8 Results from this study showed that the biaxial Schapery model can only capture the film behavior under moderate strain levels, and far away from the onset of permanent deformation.…”

Wrinkling of Orthotropic Viscoelastic Membranes

“…Kang and Im 10 proposed an iterative scheme for the finite element analysis of wrinkling in orthotropic membrane structures, which was later adopted by Gerngross and Pellegrino. [11][12][13] Alternative approaches have been proposed by Epstein and Forcinito 14 and Raible et al…”

“…Hence we eliminate ϵ y by substituting this expression into Equation 14, and obtain the effective elasticity matrix 13 for the wrinkled element…”