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NIA Seminar by James Rand |
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Date: February 14, 2006
Time: 10:30am
Location: NIA, Rm 137
A Constitutive Equation for Stratospheric Balloons
James L. Rand, Winzen Engineering, Inc.
The selection of a suitable material for use as a reliable stratospheric balloon is based on a variety of desired properties. In order to achieve the required combination of weight per unit area, strength, flexibility and toughness at low temperatures low density polyethylene has been used for the last half century. During the last decade, linear low density polyethylene has been found to have even better properties for this application. Thin films extruded from this type resin have been found to have time dependent properties which should be understood in order to make an intelligent analysis of the system. This paper describes the current effort to characterize a 38 micron coextrusion of this type of resin as a nonlinearly viscoelastic material. The resulting constitutive equation may be used to accurately describe either the time dependent creep and/or relaxation of this film when subjected to a biaxial state of stress. Recently obtained laboratory data have been used to modify an existing model of LLDPE to account for differences caused by the coextrusion process. Unfortunately, current structural analysis techniques based on linear elastic properties will predict stresses in excess of those which actually exist. This has resulted in several balloons assuming an unstable shape during deployment and pressurization. This type of instability may be avoided by eliminating excess material in the envelope and making use of the biaxial stress state which will result. Data from Dynamic Mechanical Analysis is shown to be an accurate basis for this nonlinear model.
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